Move MacOS jobs to trunk to reduce its load (#610)

Summary:
As ET is using more MacOS runners causing a long queue there https://hud.pytorch.org/metrics, an immediate mitigation step would be to move MacOS jobs to trunk to reduce its load.  I'll start working on T165554710 to consolidate MacOS jobs.

Pull Request resolved: https://github.com/pytorch/executorch/pull/610

Reviewed By: malfet

Differential Revision: D49899056

Pulled By: huydhn

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