no need for derived tags when building ephemeral images in CI

[deitch]

We were building in prbuild.yaml with the following tags:

              ${{ github.event.pull_request.head.repo.full_name }}:${{ github.event.pull_request.head.ref }}
              ${{ github.event.pull_request.head.repo.full_name }}:latest

These builds all get thrown away, for testing only. As long as the ref of the PR head is a valid tag, it worked. But if it isn't - and that can happen often - it failed. Since these are just getting thrown away, leave it as latest only.

In main build we use the actual commit, which always is a valid tag. And in release, we use semver. So those are fine.