According to [GitHub's docs][0], the `github.base_ref` property "is only
available when the event that triggers a workflow run is either
`pull_request` or `pull_request_target`.
Because of this, pushes were running the following command...
python3 Scripts/precommit.py --ref origin/
...and getting this error:
fatal: ambiguous argument 'origin/': unknown revision or path not in the working tree.
This has been broken for a long time, but it didn't cause CI to fail until
d7c33eb211.
After some discussion, we decided not to run these on pushes to `master`
or `release/*`.
(See also: [this GitHub issue][1] which asks for clarification.)
[0]: https://docs.github.com/en/actions/learn-github-actions/contexts#github-context
[1]: https://github.com/github/docs/issues/13810
This change uses a throwing computed variable which was added in Swift
5.5 and requires Xcode 13 to compile.
The default Xcode on current macOS 11 image is still 12.5.1 and won't be
updated until later this month. The newer Xcode is available on these
machines, but until October 18th we need to select it manually.
https://github.com/actions/virtual-environments/issues/4180
