Use custom test step for macOS

[dataflake]

Workaround for the many problems with the universal2-based macOS GHA runner Python builds like #181

With this change the default GHA test configuration uses a custom "Test" step for macOS runners that is hardcoded to call the respective tox environment and tacking on an environment marker. The tox configuration for each package can then react to that marker and e.g. pre-install dependencies like C-code wheels that zc.buildout cannot install itself.

Used/tested with Zope (zopefoundation/Zope#1214)

[d-maurer]

I would expect corresponding changes in (some) tox.inis.

For non Mac envs we have support for gha_test_environment and gha_test_commands; for Mac there is not support for gha_test_commands.

[dataflake]

The packages that need changes in their tox configuration will get them next time someone updates the configuration files because at that moment the tests will fail. I'm not going to go through all of them right now.

And yes, it would be nice to have macOS-specific settings for the gha--keys, but for now I will wait until there's a real need for it because changes in a packages tox.ini are not sufficient.

[icemac]

I would like to have some documentation so we do not forget about this feature. It seems only be needed when buildout is envolved, which is only the case in the zope-package and buildout-recipe templates. All other use pip for the installation.

[icemac]

Hm, I overlooked the enabled auto-merge. Maybe my suggestions still can get incorporated somehow.