Testing on macOS is always skipped in CI

[akihironitta]

🐛 Bug

When any change is added to PL codebase, tests should run in all operating systems such as ubuntu, windows and macos. However, tests on macos always get skipped.

To Reproduce

See an example in #14563 https://github.com/Lightning-AI/lightning/actions/runs/3002282279

Expected behavior

Don't skip.

Environment

macos in CI

Additional context

The skipping logic was originally added in #13661.

cc @tchaton @rohitgr7 @carmocca @akihironitta @Borda

[akihironitta]

It turns out that the issue is due to the lack of memory in macOS runners:

> changed-files
  Resolving repository path: /Users/runner/work/lightning/lightning/.
  Retrieving changes between 273a9ed8c14650c59b60c546fe53d93a4183d439 (master) → c4a680f471c3f03a20b4d59735904926189387c4 (ci/trigger-macos)
  Getting diff...
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory
  grep: out of memory

https://github.com/Lightning-AI/lightning/runs/8213161059?check_suite_focus=true#step:3:134

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There is a beta feature enabling the GitHub-hosted runners to have more resources (https://docs.github.com/en/actions/using-github-hosted-runners/using-larger-runners), but I don't think it's worth adapting it just for this issue.