Move the macOS ARM64 build job into the existing test workflows.

Also ensure it runs specifically on macOS 11.

Also build for arm64, not arm64e, as the latter isn't intended
for non-Apple-provided binaries yet (at least not on macOS 11).

Also some further refactoring to facilitate qualifying cache keys
by system version.

Also fix some unrelated docstrings that didn't justify their own change.