FIX: Use upstream/main for tools

[larsoner]

1. Don't install upstream/main for now, use stable
2. Use upstream/main rather than marsipu/pg_test

[larsoner]

@marsipu this:

1. implements the os/mne matrix, plus an old/compat 3.7 version and no opengl variants -- so 9 variants (bad) but each is only ~5 min (good) so overall a good balance of coverage versus CI burden
2. injects an ignore for the warning about not having OpenGL (hopefully temporary and we'll go with opt-in so this warning goes away)
3. revamps the tests and dev instructions so that things are simpler
4. runs the benchmarks on CIs in addition to mne-python unit tests

With this I think we can ensure that things work. Ideally we would have a nightly scheduled job that emailed us on failure. One option is to switch to Azure Pipelines, because I know how to configure such nightly emails there. It looks like there might not be a good solution for GitHub Actions currently. But in the meantime I think this PR is a step in the right direction, so feel free to merge if you're happy!

[larsoner]

@marsipu I'm going to merge this so that the opt-in behavior can be tested properly, but feel free to comment and I'm happy to open another PR.

[marsipu]

Thanks a lot @larsoner for taking care of the test-matrix and the improved benchmarks. I really like the improved cli-interface and the presentation of the results.

I am a bit confused by the results of defaults, since they should be equal to the results of use_opengl=False now, right?