ci(image-merge): apply the keepalive+ci-cache source fix to image_merge

Mirror of 8521af14 (which fixed backend_merge.yml) for image_merge.yml.
Today's master-push run 25823024353 failed the gpu-vulkan-image-merge job
with the exact same error pattern the backend merge had on v4.2.2:

  ERROR: quay.io/go-skynet/local-ai@sha256:68b22611...: not found

Same root cause: image_build.yml pushes the per-arch manifest to
quay.io/go-skynet/local-ai with push-by-digest=true (no tag), then the
merge runs minutes-to-hours later, by which time quay's per-repo manifest
GC has reaped the untagged digest from local-ai. The blob still lives in
quay's storage but local-ai@<digest> no longer resolves.

Three matching edits:

1. image_build.yml: anchor each per-arch digest into ci-cache immediately
   after the push, reusing .github/scripts/anchor-digest-in-cache.sh with
   SOURCE_IMAGE=quay.io/go-skynet/local-ai and TAG_SUFFIX defaulting to
   "-core" for the core image (matches the artifact-name convention).
2. image_merge.yml: change the quay merge source from local-ai@<digest>
   to ci-cache@<digest>. Same correctness argument as backend_merge.yml —
   the manifest content is alive in ci-cache; buildx imagetools create
   republishes it into local-ai and writes the user-facing manifest list
   pointing at it. End state in local-ai is self-contained.
3. image_merge.yml: add a sparse `actions/checkout@v6` (only
   .github/scripts) so cleanup-keepalive-tags.sh is available, plus the
   cleanup step itself with TAG_SUFFIX matching the anchor's "-core"
   placeholder.

v4.2.3's image.yml run completed successfully (~50 min between push and
merge — beat quay's GC). This commit closes the race for future releases
and master pushes regardless of run length.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>