Fix (again) snapshot uploads

The tag-release got confused when adding a commit that was not changing debian/changelog and then manually triggering a pipeline.

Triggering a pipeline manually means that only: changes: is ignored, so tag-release is run: since we're not exactly on a tagged commit the job will try to create a new tag only to discover that the tag already exists, pointing to previous commit.

To avoid that, rework the job to:

1. skip on UNRELEASED as it did before
2. fail on PLEASE SUMMARIZE as it did before
3. fetch a pre-made build if we're sitting on a tagged commit and return
4. if the changelog points to an existing tag, skip
5. run the actual build and tag if none of the previous checks matched

Tests (don't mind the failed upload stage, I've not set the OBS credentials on purpose):

• commit without debian/changelog entry
• commit with UNRELEASED in debian/changelog
• release commit
• re-running on release commit