CI Cleanup - SIGN_INSTALLER and RELEASE_BRANCH_BUILD

[krzywon]

The RELEASE_BRANCH_BUILD and SIGN_INSTALLER CI environment variables could use some cleanup. The release variable should be more generic, with nothing related to v6.1. The installer signing variable could potentially use the release variable as part of the definition.

Note - I'm not sure the latter is possible, so this may not be possible.

[llimeht]

Addressing this probably comes as part of #3642 - factoring out these variables would be reasonable at this stage.

I think something like (usual caveat being that this is very hard to test):

  RELEASE_BRANCH_BUILD: ${{
      (
        (github.event_name == 'push' && startsWith(github.ref, 'refs/heads/release')) ||
        (github.event_name == 'push' && startsWith(github.ref, 'refs/tags/v')) ||
        (github.event_name == 'pull_request' && startsWith(github.base_ref, 'refs/heads/release'))
      ) && 'true' || '' }}

      SIGN_INSTALLER: ${{
        (
          (! github.event.repository.fork) &&
          (! github.event.pull_request.head.repo.fork) &&
          (startsWith(github.ref, 'refs/tags/v') ||
          startsWith(github.ref, 'refs/heads/release'))
        ) && 'true' || '' }}

That does make all branches starting with release special - not just the release[-_]x.y.z branches; likewise it makes all tags starting with v special. Is that ok?

If one is actually trying to debug the signing process for any reason, then that definition of SIGN_INSTALLER will need disabling because it doesn't sign until after merging the PR into the release branch.