Clean registry buildcache when PR merged or closed

**Is your feature request related to a problem? Please describe.**
We write the buildcache for pull requests to the registry as tags for the buildcache image at https://eicweb.phy.anl.gov/containers/eic_container/container_registry/143. This accumulates without a clear way to clean up since the buildcache tags don't have a timestamp set that could be used for automatic eviction. This also affects ghcr.io.

**Describe the solution you'd like**
When a pull request is merged, we should explicitly remove the buildcache tags from the registries (both eicweb and ghcr.io).

**Describe alternatives you've considered**
An alternative approach is to have a periodic scheduled workflow that removes the buildcache entries that do not have a corresponding open pull request.