release: handle multiple branches

[kousu]

Update the actions workflows, both sync-upstream.yml and release.yml, so that we can maintain an arbitrary number of feature branches, starting with:

• release-action (release-action #2)
• git-annex (git-annex #1)
• cornerstone (cornerstone: Display NIfTI and dicom image previews #30)

These should all be regularly rebased on upstream/main, and, except for release-action itself, built and pushed to /releases when a new upstream release comes out.

Also, make sure to handle

• git-annex-cornerstone (cornerstone + git-annex #24)

which should be the merge of both of those (so, probably, cornerstone rebased on git-annex)

All of these steps should run as separate jobs so it's easy to see what went wrong when something goes wrong. The problem with the current workflows is they do everything in one shot, so it's difficult to scale and even more difficult to do #17.

[kousu]

This might call for pulling out https://stacked-git.github.io/