ci: build last four minor R versions instead of only the latest

Derive the latest patch release of each of the last four minor R versions
from posit's versions.json and build any that are missing from S3,
so older supported minors stay available rather than only the newest stable.

Replace the awk regex existence check with a literal index() match;
the unescaped version dots made e.g. 4.4.3 spuriously match 4.3.3.

.crow/build.yaml

@@ -114,15 +114,26 @@ steps:
           kubernetes.io/arch: "${ARCH}"
 
 
-  - name: Get latest R version
+  - name: Get R versions to build
     image: reg.devxy.io/docker.io/library/alpine:3.23
     privileged: true
     commands:
       - ip link set dev eth0 mtu 1280 2>/dev/null || true
       - for i in 1 2 3 4 5; do apk add -q --no-cache curl jq && break; sleep 5; done
-      - curl -sf https://formulae.brew.sh/api/formula/r.json | jq -er '.versions.stable' > r-version-to-build.txt
-      # - echo "4.1.3" > r-version-to-build.txt
-      - cat r-version-to-build.txt
+      # Latest patch release of each of the last 4 minor R versions (e.g. 4.6.0, 4.5.3, 4.4.3, 4.3.3).
+      - |
+          curl -sf https://cdn.posit.co/r/versions.json | jq -r '
+            .r_versions
+            | map(select(test("^[0-9]+\\.[0-9]+\\.[0-9]+$")))
+            | group_by(split(".")[0:2] | join("."))
+            | map(max_by(split(".") | map(tonumber)))
+            | sort_by(split(".") | map(tonumber))
+            | reverse
+            | .[0:4]
+            | .[]' > r-versions-to-build.txt
+      # To pin specific versions for testing, overwrite the file, e.g.:
+      # - printf '4.1.3\n' > r-versions-to-build.txt
+      - cat r-versions-to-build.txt
     backend_options:
       kubernetes:
         resources:
@@ -136,19 +147,23 @@ steps:
           kubernetes.io/arch: "${ARCH}"
 
 
-  - name: Check if files for R version already exist in s3
+  - name: Check which R versions already exist in s3
     image: reg.devxy.io/docker.io/library/alpine:3.23
     privileged: true
     commands:
       - ip link set dev eth0 mtu 1280 2>/dev/null || true
       - for i in 1 2 3 4 5; do apk add -q --no-cache curl && break; sleep 5; done
       - |
-          VERSION=$(cat r-version-to-build.txt)
-          curl -s "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
-            grep -oE '<Key>[^<]+</Key>' | sed 's/<[^>]*>//g' | \
-            awk -v arch="${ARCH_ID}" -v ver="${VERSION}" \
-              '$0 ~ arch && $0 ~ ver {print substr($NF, 3, 5)}' > r-version-s3.txt
-      - cat r-version-s3.txt
+          LISTING=$(curl -s "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
+            grep -oE '<Key>[^<]+</Key>' | sed 's/<[^>]*>//g')
+          : > r-versions-existing.txt
+          for VERSION in $(cat r-versions-to-build.txt); do
+            if printf '%s\n' "$LISTING" | \
+              awk -v arch="${ARCH_ID}" -v ver="$VERSION" 'index($0, arch) && index($0, ver) { found=1 } END { exit !found }'; then
+              echo "$VERSION" >> r-versions-existing.txt
+            fi
+          done
+      - echo "Already present in s3:"; cat r-versions-existing.txt
     backend_options:
       kubernetes:
         nodeSelector:
@@ -164,9 +179,20 @@ steps:
     commands: |
       ip link set dev eth0 mtu 1280 2>/dev/null || true
       # docker info
-      if ! grep -qF "$(cat r-version-to-build.txt)" r-version-s3.txt; then
+      TO_BUILD=""
+      for VERSION in $(cat r-versions-to-build.txt); do
+        if grep -qxF "$VERSION" r-versions-existing.txt; then
+          echo "R $VERSION already exists for ${PLATFORM} (${ARCH_ID}), skipping"
+        else
+          TO_BUILD="$TO_BUILD $VERSION"
+        fi
+      done
+      if [ -n "$TO_BUILD" ]; then
         for i in 1 2 3 4 5; do apk add -q --no-cache make just docker-compose && break; sleep 5; done
-        just build-r-${PLATFORM} $(cat r-version-to-build.txt)
+        for VERSION in $TO_BUILD; do
+          echo "Building R $VERSION for ${PLATFORM}"
+          just build-r-${PLATFORM} "$VERSION"
+        done
       fi
     backend_options:
       kubernetes: