fix(ci): build per-minor binaries with a per-minor R library

The sensitive-only multi-R build passes ran under each minor's R but loaded
bincraft from the shared R_LIBS_USER=/mnt/cache/R-pkgs cache, which is compiled
under the primary R and fails to load under a newer minor (e.g. 4.6:
"undefined symbol: SETLENGTH") — so those minor builds silently no-op'd.

Give each non-primary minor its own R_LIBS_USER=/mnt/cache/R-pkgs-<minor> and
install bincraft 4.2.1 there (built for that minor); the rest of bincraft's
deps load from the per-version /opt/R/<minor>/lib. Applied to the
build-all-versions and process-updates sensitive-only passes.

.crow/build-all-versions-amd64.yaml

@@ -75,7 +75,10 @@ steps:
           RMINOR=$(echo "$RV" | cut -d. -f1-2)
           [ "$RMINOR" = "$PRIMARY_MINOR" ] && continue
           echo "=== R-minor-sensitive pass under R $RV ==="
-          $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- "$(dirname "$RBIN")/Rscript" local/build-all.R --sensitive-only $SPLIT_INTO $SPLIT_INDEX $NCPUS 2>&1 || true
+          LIB="/mnt/cache/R-pkgs-$RMINOR"
+          mkdir -p "$LIB"
+          R_LIBS_USER="$LIB" "$(dirname "$RBIN")/R" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.1")' || true
+          R_LIBS_USER="$LIB" $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- "$(dirname "$RBIN")/Rscript" local/build-all.R --sensitive-only $SPLIT_INTO $SPLIT_INDEX $NCPUS 2>&1 || true
         done
       # archive missed packages
       - /opt/R/$R_VERSION/bin/R -q -e "bincraft::process_unarchived_pkgs(paste(Sys.getenv('OS'), Sys.getenv('OS_VERSION')), Sys.getenv('ARCH'), workers = $NCPUS)"