Apply pre-release label only when the commit shasum does not match main

[mksanger]

Allows releases to be marked as release automatically (rather than prerelease)

[ghost]

I'm not sure prerelease can ever be false, regardless of it being refs/heads/master or refs/heads/main. From the GitHub Actions Context docs, github.ref won't refer to the branch name when the workflow is triggered by a tag:

The branch or tag ref that triggered the workflow run. For workflows triggered by push, this is the branch or tag ref that was pushed. For workflows triggered by pull_request, this is the pull request merge branch. For workflows triggered by release, this is the release tag created. For other triggers, this is the branch or tag ref that triggered the workflow run. This is only set if a branch or tag is available for the event type. The ref given is fully-formed, meaning that for branches the format is refs/heads/<branch_name>, for pull requests it is refs/pull/<pr_number>/merge, and for tags it is refs/tags/<tag_name>. For example, refs/heads/feature-branch-1.

Since the workflow is only triggered by tag, github.ref would always be refs/tags/<tag_name>, making all releases pre-releases.

[mksanger]

See https://github.com/mksanger/test_release for a test of the new version

[ghost]

That works, but it comes with the issue that if you try to tag another commit than the HEAD of main, it'll be marked as a prerelease. That's probably not an issue since we only merge to master on releases anyway, and it shouldn't be hard to change a release type after it being published.