When preparing a portability report such as https://groups.google.com/g/sage-release/c/GOGWk66zaCQ/m/dBQww8WNAAAJ, I have found that the output of tox.yml has gotten too big to be comfortably inspected in one go.
So we split out everything macOS into a separate workflow.

Preparation for #33062 and #32570

Depends on #34017

CC: @dimpase @kliem

Component: porting

Author: Matthias Koeppe

Branch/Commit: aa25405

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33788