remove need to build xtask from CI

[davidhewitt]

After discussions in #2619 I think we're leaning towards preferring nox over xtask for simplicity. This moves the coverage implementation from xtask to nox, and also removes xtask from the main CI runs. Hopefully shortens CI build times a bit.

Potentially a follow-up to move all of the xtask ci logic to a nox -s ci job.

[adamreichold]

I think the grouping does not work as intended yet?