run nox -s clippy-all against free-threaded build in CI

[ngoldbaum]

While working on #4588 I realized the clippy CI job doesn't run against the free-threaded build. Rather than adjusting the clippy CI job to install a free-threaded Python, I thought it was easier to update the existing free-threaded test to do this.

[davidhewitt]

I don't think this is needed:

• the clippy-all job is to run over (almost) all Python versions, and it's suuuper slow
• the clippy job is already run as part of the nox command just below (clippy in in the default sessions)

So clippy should already be running and this is just redundant extra work, I believe. (Covered by the normal clippy ubuntu job.)

[ngoldbaum]

OK, fair enough. Thanks for explaining.