Don't attempt to test CPython 3.8 wheels on Apple Silicon.

[joerick]

I've added a warning for users, so they know that it doesn't get tested.
See discussion in #1169 for the reason why.

Fixes #1168, fixes #1169

[joerick]

I've verified the test passes on my local M1-

$ CIBW_PLATFORM=macos pytest test/test_macos_archs.py -k test_cp38_arm64_testing
============================= test session starts ==============================
platform darwin -- Python 3.11.0b3, pytest-7.1.2, pluggy-1.0.0
rootdir: /Users/joerick/Projects/cibuildwheel, configfile: pyproject.toml
plugins: xdist-2.5.0, forked-1.4.0, timeout-2.1.0
collected 6 items / 5 deselected / 1 selected                                  

test/test_macos_archs.py .                                               [100%]

======================= 1 passed, 5 deselected in 31.37s =======================