ci(release-image): Skip digest upload when not pushing images

After #992, builds without registry credentials skip Docker image output
but still extract binary artifacts. However, we were still trying to
upload digests for images that weren't created. Add conditional check
to only upload digests when actually pushing to registry.

.forgejo/workflows/release-image.yml

@@ -249,6 +249,7 @@ jobs:
           if-no-files-found: error
 
       - name: Upload digest
+        if: ${{ env.BUILTIN_REGISTRY_ENABLED == 'true' }}
         uses: forgejo/upload-artifact@v4
         with:
           name: digests-${{ matrix.slug }}