Use only cmake for MacOS CI to save capacity (#613)

Summary:
This change updates the gather script to:

1. Use only cmake for MacOS CI to save capacity. This cuts the number of MacOS job in half
2. Fix the wrong and confusing reference of `linux.2xlarge` on MacOS jobs

Pull Request resolved: https://github.com/pytorch/executorch/pull/613

Reviewed By: seemethere

Differential Revision: D49919115

Pulled By: huydhn

fbshipit-source-id: c6b8619153959259786e8c2adb3e721934c3841e