Fix failing `system-test-macos-x86_64` test (#3218)
## Summary
GitHub recently changed `macos-latest` to `macos-12`, so the x86 test
started failing. I've migrated the tests to use dedicated labels, rather
than the aliases.
See:
https://github.blog/changelog/2024-01-30-github-actions-macos-14-sonoma-is-now-available/
