fix(ci): detect existing R versions via public object HEAD probe

The existing-version check listed the bucket with an anonymous curl, but
Backblaze B2 requires authentication for the list-bucket API, so it returned
AccessDenied and r-versions-existing.txt was always empty -> every version
was rebuilt despite already existing in s3. Signing the request is not
workable here: the B2 credentials are scoped to the plugin-s3 image in crow
and are not injected into this plain alpine step.

Individual artifacts are public-read, so probe each expected object URL with
an anonymous HEAD instead. The filename mirrors what nfpm produces (version
and release pinned to 1): r-<ver>_1_<arch>.{apk,deb} for alpine/ubuntu and
R-<ver>-1-1.<arch>.rpm for el*. Verified against the live bucket for all
three packager families.

Shell variables use bare $name rather than ${name}: crow runs its own
${VAR} substitution over the commands before the shell executes, so a
${VAR} naming a shell var (BASE, VERSION) was blanked to empty -- only
matrix vars resolve at that stage. Build the filename with printf to avoid
the ${VERSION}_1 brace requirement entirely.

.crow/build.yaml

@@ -158,28 +158,29 @@ steps:
   - name: Check which R versions already exist in s3
     image: reg.devxy.io/docker.io/library/alpine:3.23
     privileged: true
-    environment:
-      AWS_ACCESS_KEY_ID:
-        from_secret: B2_S3_ACCESS_KEY_ID
-      AWS_SECRET_ACCESS_KEY:
-        from_secret: B2_S3_SECRET_KEY
     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
       - |
-          # Backblaze B2 requires authentication for the list-bucket API (anonymous
-          # GET works only for individual public-read objects), so the request must be
-          # SigV4-signed with the same credentials used for the upload step. Without
-          # this the listing returns AccessDenied, r-versions-existing.txt stays empty,
-          # and every version is rebuilt even though it already exists.
-          LISTING=$(curl -s --aws-sigv4 "aws:amz:eu-central-003:s3" \
-            --user "${AWS_ACCESS_KEY_ID}:${AWS_SECRET_ACCESS_KEY}" \
-            "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
-            grep -oE '<Key>[^<]+</Key>' | sed 's/<[^>]*>//g')
+          # Backblaze B2's list-bucket API requires authentication, but the upload
+          # credentials are scoped to the plugin-s3 image and aren't available here.
+          # Individual objects are public-read, so probe each expected artifact URL
+          # 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*. 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-*) 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 $(cat r-versions-to-build.txt); do
-            if printf '%s\n' "$LISTING" | \
-              awk -v arch="${ARCH_ID}" -v ver="$VERSION" 'index($0, arch) && index($0, ver) { found=1 } END { exit !found }'; then
+            URL="$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
           done