fix(ci): use bare shell vars in existing-version check (crow ${VAR} substitution)

The merged HEAD-probe check still rebuilt everything: crow runs its own
${VAR} substitution over the commands before the shell executes, so the
shell variables ${BASE} and ${VERSION} were blanked to empty (only matrix
vars resolve at that stage). Every probed URL was therefore malformed and
returned non-200, leaving r-versions-existing.txt empty.

Use bare $name for shell variables and build the filename with printf, which
avoids the ${VERSION}_1 brace requirement entirely. Matrix vars (${PLATFORM},
${PLATFORM_ID}, ${ARCH_ID}) keep braces since crow resolves those correctly.
Verified URL construction against the live bucket for apk, deb and rpm.

.crow/build.yaml

@@ -168,16 +168,18 @@ steps:
           # with an anonymous HEAD instead of listing the bucket. The filename mirrors
           # what nfpm produces in builder/package.${PLATFORM} (version/release pinned
           # to 1): r-<ver>_1_<arch>.{apk,deb} for alpine/ubuntu, R-<ver>-1-1.<arch>.rpm
-          # for el*.
+          # for el*. Shell variables must use bare $name, not ${name}: crow performs
+          # its own ${VAR} substitution on these commands before the shell runs, so
+          # ${VAR} for a shell var would be blanked out (only matrix vars resolve there).
           case "${PLATFORM}" in
-            alpine-*) FILE="r-VER_1_${ARCH_ID}.apk" ;;
-            ubuntu-*) FILE="r-VER_1_${ARCH_ID}.deb" ;;
-            *)        FILE="R-VER-1-1.${ARCH_ID}.rpm" ;;
+            alpine-*) FMT="r-%s_1_%s.apk" ;;
+            ubuntu-*) FMT="r-%s_1_%s.deb" ;;
+            *)        FMT="R-%s-1-1.%s.rpm" ;;
           esac
           BASE="https://s3.eu-central-003.backblazeb2.com/devxy-r-builds/${PLATFORM_ID}"
           : > r-versions-existing.txt
           for VERSION in $(cat r-versions-to-build.txt); do
-            URL="${BASE}/$(echo "$FILE" | sed "s/VER/${VERSION}/")"
+            URL="$BASE/$(printf "$FMT" "$VERSION" "${ARCH_ID}")"
             if [ "$(curl -s -o /dev/null -w '%{http_code}' -I "$URL")" = "200" ]; then
               echo "$VERSION" >> r-versions-existing.txt
             fi