os/mac/xquartz: fix handling of XQuartz 2.8.0+

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

This should go completely. We maybe need to properly deprecate ENV.x11 first though, not entirely sure.

Fixes #12115.

[BrewTestBot]

Review period skipped due to critical label.

[Bo98]

I think yes, I'll remove this entirely but in a separate PR.