fix(ci): declare FORCE_REBUILD as a UI variable and make it take effect

Follow-up to #11, which landed the trigger-form handling but not the rest.
Three things still stood between FORCE_REBUILD and an actual republish.

Declare FORCE_REBUILD as a top-level variable so the UI offers it as a
dropdown on a manual run, rather than requiring a hand-typed key.

Test the value rather than emptiness. A declared variable substitutes its
default, so the `[ -n ... ]` check from #11 would have seen the literal
string "false" and rebuilt every platform on every cron run.

Set overwrite: true on the upload. plugin-s3 skips an object that already
exists when overwrite is false, logging "File already exists, skipping
upload" and still exiting green, so a forced rebuild would have recompiled
every package and published none of them. Normal runs are unaffected, since
dedup means only not-yet-published versions are ever built.

Also drop the Net debug step, the commented-out Pull upstream step and a
stray `# docker info`. The MTU workaround the debug step was diagnosing
stays, as does the commented-out alpine-320 matrix entry, which records a
deliberately disabled platform rather than dead scaffolding.

.crow/build.yaml

@@ -1,6 +1,18 @@
 when:
   event: [cron, manual]
 
+variables:
+  FORCE_REBUILD:
+    description: >-
+      Rebuild every targeted R version even when the artifact is already in the
+      bucket. Needed after a builder-image change that alters the resulting R
+      binary, because artifact filenames carry only the R version and would
+      otherwise dedup to the stale build forever.
+    options:
+      - "false"
+      - "true"
+    default: "false"
+
 matrix:
   include:
     - PLATFORM: rhel-9
@@ -82,46 +94,6 @@ matrix:
       ARCH_ID: x86_64
 
 steps:
-  # Ensures that we're always using the latest available upstream build chain
-  # - name: Pull upstream
-  #   image: reg.devxy.io/docker.io/library/alpine:3.20
-  #   commands:
-  #     - apk add -q --no-cache git
-  #     - git remote add upstream https://github.com/cynkra/r-builds.git
-  #     - git config --global user.email "you@example.com"
-  #     - git config --global user.name "Your Name"
-  #     - git config pull.ff only
-  #     - git fetch upstream
-  #     - git merge upstream/main --allow-unrelated-histories
-  #.
-
-  - name: Net debug
-    image: reg.devxy.io/docker.io/library/alpine:3.23
-    privileged: true
-    commands:
-      - ip link set dev eth0 mtu 1280 2>/dev/null || true
-      - echo "=== pod interface ==="
-      - cat /sys/class/net/eth0/mtu
-      - ip -o link show eth0
-      - echo "=== route ==="
-      - ip route
-      - echo "=== resolv.conf ==="
-      - cat /etc/resolv.conf
-      - for i in 1 2 3; do apk add -q --no-cache iputils && break; sleep 5; done || echo "apk failed; falling back to busybox ping (no -M do)"
-      - |
-          echo "=== PMTUD probes (DF=1) to dl-cdn.alpinelinux.org ==="
-          for sz in 1472 1392 1352 1252 1200; do
-            echo "--- payload=$sz (wire ~$((sz+28))) ---"
-            ping -c 2 -W 3 -M do -s $sz dl-cdn.alpinelinux.org || echo "(size=$sz failed)"
-          done
-      - ping -c 3 -W 5 dl-cdn.alpinelinux.org || true
-      - wget --tries=1 --timeout=10 -O /dev/null -nv https://dl-cdn.alpinelinux.org/ 2>&1 || true
-    backend_options:
-      kubernetes:
-        nodeSelector:
-          kubernetes.io/arch: "${ARCH}"
-
-
   - name: Get R versions to build
     image: reg.devxy.io/docker.io/library/alpine:3.23
     privileged: true
@@ -178,22 +150,23 @@ steps:
           esac
           BASE="https://s3.eu-central-003.backblazeb2.com/devxy-r-builds/${PLATFORM_ID}"
           : > r-versions-existing.txt
-          # Escape hatch for when the builder image itself changes in a way that
-          # alters the resulting R binary (a newly enabled graphics device, a
-          # different BLAS). The artifact filename carries only the R version, so
-          # dedup would otherwise keep the stale binaries forever. Uploads
-          # overwrite in place.
-          #
-          # The two ways to trigger a manual run pass the flag differently:
-          #   crow pipeline create --branch main --var FORCE_REBUILD=1
-          #     -> a crow substitution variable, read below as ${FORCE_REBUILD},
-          #        which crow blanks out when it was not supplied
-          #   crow pipeline start -p FORCE_REBUILD=1
+          # FORCE_REBUILD is declared as a top-level variable, so the UI offers it
+          # as a dropdown on a manual run. The two trigger paths deliver it
+          # differently, so accept either:
+          #   UI, or crow pipeline create --var FORCE_REBUILD=true
+          #     -> a substitution variable, expanded into ${FORCE_REBUILD} below
+          #   crow pipeline start -p FORCE_REBUILD=true
           #     -> injected into the step environment, read as bare $FORCE_REBUILD
-          # Accept either, so the flag cannot be silently ignored.
-          FORCE_REBUILD_VAR="${FORCE_REBUILD}"
-          if [ -n "$FORCE_REBUILD_VAR" ] || [ -n "$FORCE_REBUILD" ]; then
-            echo "FORCE_REBUILD is set, ignoring artifacts already in the bucket"
+          # The declared default is the string "false", so test the value rather
+          # than emptiness; an empty value means neither path supplied one.
+          FORCE_VALUE="${FORCE_REBUILD}"
+          [ -n "$FORCE_VALUE" ] || FORCE_VALUE="$FORCE_REBUILD"
+          case "$FORCE_VALUE" in
+            true|TRUE|True|1|yes|YES) FORCE_ON=1 ;;
+            *)                        FORCE_ON="" ;;
+          esac
+          if [ -n "$FORCE_ON" ]; then
+            echo "FORCE_REBUILD=$FORCE_VALUE, ignoring artifacts already in the bucket"
           else
             for VERSION in $(cat r-versions-to-build.txt); do
               URL="$BASE/$(printf "$FMT" "$VERSION" "${ARCH_ID}")"
@@ -217,7 +190,6 @@ steps:
       DOCKER_HOST: tcp://docker:2375
     commands: |
       ip link set dev eth0 mtu 1280 2>/dev/null || true
-      # docker info
       TO_BUILD=""
       for VERSION in $(cat r-versions-to-build.txt); do
         if grep -qxF "$VERSION" r-versions-existing.txt; then
@@ -263,7 +235,11 @@ steps:
         from_secret: B2_S3_ACCESS_KEY_ID
       secret_key:
         from_secret: B2_S3_SECRET_KEY
-      overwrite: false
+      # Must stay true for FORCE_REBUILD to mean anything: with overwrite=false
+      # the plugin logs "File already exists, skipping upload" and still exits
+      # green, so a rebuild would be discarded. Normal runs are unaffected,
+      # since dedup means only not-yet-published versions are ever built.
+      overwrite: true
     volumes:
       - ci-tmp-rwx:/tmp # agent on k8s
       # - /tmp:/tmp # agent on VM