Update GitHub Actions workflow

[scpeters]

We have a GitHub actions workflow that calls brew test-bot on all pull request and master branch commits, but it's behavior has changed from what I initially intended. I originally intended that the Actions CI should install any bottles that have changed and run brew test and brew audit, similar to the project-install-homebrew.bash script in release-tools.

1. All master branch builds fail since it is unable to detect variables showing what has changed since the last job.
2. All pull request builds seem to be rebuilding bottles instead of testing the existing ones.

In both cases, we should only test formulae that have bottles, and if so, install, test, and audit them.