Automatically re-tag GitHub Action version

[dennis-sig]

GitHub Marketplace requires explicit versions and tags, they don't allow continuous delivery. Some actions, for example https://github.com/marketplace/actions/git-version, use semver like 2.4.0. This seems nice, but it's actually not because everyone needs to change their GitHub Action configuration whenever a new version is released. Most other actions "cheat". See for example https://github.com/marketplace/actions/knative-service-manager. They use a ridiculously generic version like "v1", then then move the tag when they want to release. We should follow this approach.

Of course we can move the tag by hand, but that's error prone. Ideally we would re-tag after a successful commit to main.