Support MacOS Big Sur on CI by setting the appropriate Qt env var

[rahulporuri]

Similar to https://github.com/enthought/traitsui/pull/1789/ I suspect this is the reason why the cron job - https://github.com/enthought/traits/runs/4602763635?check_suite_focus=true - failed.

I'm re-ran the cron job from this branch - https://github.com/enthought/traits/actions/runs/1609786940

Checklist

• [ ] ~Tests~
• [ ] ~Update API reference (docs/source/traits_api_reference)~
• [ ] ~Update User manual (docs/source/traits_user_manual)~
• [ ] ~Update type annotation hints in traits-stubs~

[corranwebster]

The error looks unrelated to the Mac issue - for some reason unittest couldn't even import traits. It didn't look like it got to the point where it was trying to do anything with Qt.

[mdickinson]

I don't think this change is needed. The difference with TraitsUI is that TraitsUI is using the EDM version of PySide2, while here we're using the PyPI version. That version doesn't seem to need the environment variable set.

It looks to me as though the cron job failure is unrelated.