chore: ci: move `macOS aarch64` from merge queue to master check

This PR stops running the `macOS aarch64` build in the merge queue and on `merge-ci` PRs, which mirror it, and runs it on pushes to `master` instead. Nightlies and `macos-arm-ci` PRs keep running it as before. It was the last job to finish in 78% of recent merge queue runs, taking a median 26 minutes against 13 for `Linux release` on the self-hosted runner, so a merge queue run should take about half as long. A macOS-only build failure is now reported on `master` rather than blocking the merge.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018sjxekJp69XeWYuZtGdoQH