CI: Workaround CodeQL pip install issues by changing Ubuntu version.

[carlosperate]

I'm not too sure why in the GH Actions runner ubuntu-latest, which has been recently updated from 20.04 to 22.04, the pip resolver struggles to download wheels and (when given some help by manually installing some subdependencies at a version we know should work) it ends up with a large recursion of trying to find dependency versions that match all requirements.

And when ubuntu-latest is used with the Python Github Action it does work, not sure why the default system Python/pip fails.

So, specifying ubuntu-20.04 as the runner resolves/worksaround the issue. Also, as we will be updating the Mu dependencies soon, there is not much point spending more time on this unless it starts affecting users pip installing Mu.

[carlosperate]

@ntoll @tjguk this is the last PR to get the CI back to all green, so I'll merge as soon as all the tests pass, and any work in progress can be rebased to get a more accurate CI pass/fail output.

[tjg-global]

Definite +1 from me. Thanks, @carlosperate

[carlosperate]

Great, merging now! 🎉