feat(rebuild): allow rebuilding for an explicit R minor

The matrix pins each slot's primary R minor, and the weekly cron only
ever builds that one. Non-primary minors are filled in incidentally, by
process-cran-updates looping the installed interpreters for new and
updated packages, so older packages accumulate a backlog there.

That is why rlang exists for 4.4 and 4.5 on amd64/resolute but not 4.6,
and why an R 4.6.1 client got a 4.5.3 binary and 'undefined symbol:
SETLENGTH'. 2709 records across the 16 slots are in that state.

rebuild_r_version selects the interpreter; rebuild-missing.R derives its
target slot from the R it runs under, so that is the whole mechanism.
'matrix' keeps today's behaviour. An R minor absent from the image fails
the step with the available list rather than silently rebuilding the
wrong slot.

.crow/weekly-rebuild-missing.yaml

@@ -20,6 +20,19 @@ variables:
   # Gates this pipeline. A manual pipeline creation instantiates every file in
   # .crow/, and a declared default is applied even when the run never passed
   # this variable, so the default must be a value that matches no matrix row.
+  # Which R minor to rebuild for. The matrix pins each slot's primary minor,
+  # which is the only one the weekly cron ever builds - that is why non-primary
+  # minors accumulate a backlog and why an ABI-risky package can exist for 4.4
+  # and 4.5 but not 4.6. Override this to work that backlog off.
+  rebuild_r_version:
+    description: "R version to rebuild with: 'matrix' for each slot's primary, or an explicit x.y.z."
+    options:
+      - matrix
+      - 4.6.0
+      - 4.5.3
+      - 4.4.3
+    default: matrix
+
   weekly_rebuild_missing:
     description: "Manual run target: a specific <os>-<arch>, 'all' to run every os/arch in parallel, or 'none' to run nothing."
     options:
@@ -410,6 +423,7 @@ steps:
       UVR_PACKAGES_DIR: /mnt/cache/uvr/packages
       R_LIBS_USER: /mnt/cache/R-pkgs
       R_VERSION: ${R_VERSION}
+      REBUILD_R_VERSION: ${rebuild_r_version}
       CCACHE_DIR: /mnt/cache/ccache
       PLATFORM: ${OS}
       ARCH: ${ARCH}
@@ -424,12 +438,26 @@ steps:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - mkdir -p /mnt/cache/uvr/cache /mnt/cache/uvr/packages /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
-      - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R
-      - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
+      # `matrix` keeps the slot's primary minor; anything else overrides it.
+      # rebuild-missing.R derives its target slot from the R it runs under, so
+      # selecting the interpreter is all that is needed to fill a minor's gap.
+      - |
+        if [ "$REBUILD_R_VERSION" = "matrix" ] || [ -z "$REBUILD_R_VERSION" ]; then
+          RV="$R_VERSION"
+        else
+          RV="$REBUILD_R_VERSION"
+        fi
+        if [ ! -x "/opt/R/$RV/bin/R" ]; then
+          echo "R $RV is not installed in this image; available: $(ls /opt/R)" >&2
+          exit 1
+        fi
+        echo "Rebuilding with R $RV"
+      - /opt/R/$RV/bin/Rscript local/install-bincraft.R
+      - /opt/R/$RV/bin/R -q -e 'packageVersion("bincraft")'
       - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
-      - UVR_R_BIN=/opt/R/$R_VERSION/bin/R local/uvr-install.sh httr2
-      - /opt/R/$R_VERSION/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
-      - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$R_VERSION/bin/Rscript local/rebuild-missing.R $SPLIT_INTO $SPLIT_INDEX $REBUILD_BUDGET_HOURS 2>&1
+      - UVR_R_BIN=/opt/R/$RV/bin/R local/uvr-install.sh httr2
+      - /opt/R/$RV/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
+      - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$RV/bin/Rscript local/rebuild-missing.R $SPLIT_INTO $SPLIT_INDEX $REBUILD_BUDGET_HOURS 2>&1
     backend_options:
       docker:
         resources: