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
(curl saw --user ":").

Individual artifacts are public-read, so probe each expected object URL with
an anonymous HEAD instead of listing the bucket. The filename mirrors what
nfpm produces (version/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.

.crow/build.yaml

@@ -162,12 +162,23 @@ steps:
       - 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
       - |
-          LISTING=$(curl -s "https://s3.eu-central-003.backblazeb2.com/devxy-r-builds?prefix=${PLATFORM_ID}/" | \
+          # Backblaze B2's list-bucket API requires authentication, but the upload
-            grep -oE '<Key>[^<]+</Key>' | sed 's/<[^>]*>//g')
+          # 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.${PLATFORM} (version/release pinned
+          # to 1): r-<ver>_1_<arch>.{apk,deb} for alpine/ubuntu, R-<ver>-1-1.<arch>.rpm
+          # for el*.
+          case "${PLATFORM}" in
+            alpine-*) FILE="r-VER_1_${ARCH_ID}.apk" ;;
+            ubuntu-*) FILE="r-VER_1_${ARCH_ID}.deb" ;;
+            *)        FILE="R-VER-1-1.${ARCH_ID}.rpm" ;;
+          esac
+          BASE="https://s3.eu-central-003.backblazeb2.com/devxy-r-builds/${PLATFORM_ID}"
           : > r-versions-existing.txt
           for VERSION in $(cat r-versions-to-build.txt); do
-            if printf '%s\n' "$LISTING" | \
+            URL="${BASE}/$(echo "$FILE" | sed "s/VER/${VERSION}/")"
-              awk -v arch="${ARCH_ID}" -v ver="$VERSION" 'index($0, arch) && index($0, ver) { found=1 } END { exit !found }'; then
+            if [ "$(curl -s -o /dev/null -w '%{http_code}' -I "$URL")" = "200" ]; then
               echo "$VERSION" >> r-versions-existing.txt
             fi
           done