fix: remove registry build cache — Forgejo rejects large layer blobs

Even mode=min still hits the registry's blob size limit (400 Bad Request).
The local runner's Docker daemon layer cache is sufficient for fast
incremental builds without needing a separate registry cache tag.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -142,5 +142,3 @@ jobs:
           push: true
           tags: ${{ steps.tags.outputs.value }}
           build-args: BUILD_VERSION=${{ steps.tags.outputs.build_version }}
-          cache-from: type=registry,ref=${{ env.IMAGE }}:cache
-          cache-to: type=registry,ref=${{ env.IMAGE }}:cache,mode=min