ci: run macOS Tests for in-repo pull requests on the Mac

[datlechin]

Summary

• macOS Tests for a pull request from a branch in this repository runs on mac-mini-m4pro. Fork PRs, pushes to main and release/**, tags and iOS stay on xcode-27.
• MAC_RUNNER=github moves it back to xcode-27, like Release Build.
• The runner now runs as the standard user runner in its own login session; the ci rule says so and how to bring it back after a restart.
• Why: v0.83.0-beta.1 waited 35 to 50 min for a hosted macOS slot while PR runs held all five.

Test plan

• actionlint clean.
• This PR's own macOS Tests run on the Mac, Unit and UI plans, in the runner session.