CI: Prevent a double automatic release

[jiribenes]

In .github/workflows/autorelease.yml, before making a new release:

1. get the version number of the current release
2. find the appropriate git tag for the current release
3. check that the git tag points to something else than master does
   • if not, exit successfully: no release will be made
   • also, do this check only if we're on the automatic release, not if the workflow was manual
4. continue with releasing (bumping the version, etc...)

We could also be even smarter and check if the changes since the last release are more than just changes in .github/ :)