chore(ci): cache pip deps in GitHub Actions cache

[miketheman]

Refs: https://github.com/marketplace/actions/setup-python#caching-packages-dependencies

[miketheman]

Per conversation with @ichard26 , this likely will not provide material benefit to speed up CI.

There's no dependency version specification for nox - it's get latest always.

Without a reqirements.txt/pyproject.toml with versions to hash against, the nox version and its dependencies would never be updated, which is also undesirable.

[ichard26]

For context, I was thinking out loud on Discord about ways to speed up the test suite.¹ One of those ways is reducing the unnecessary build dependency installations the functional tests perform. I wired up some janky logging, and I observed ~80 seconds spent installing build dependencies (excluding the keyring tests) on a 5-core ~4 minute run. The logging was slightly broken as it reported the installs occuring the test setup phase, hence this drive-by PR, but I explained that these installs are occuring during test execution where the pip cache is disabled anyway.

Furthermore, CI setup also won't benefit from caching as explained by Mike earlier.

Footnotes

1. I will probably open an issue for this at some point. I just like to share my half-thought-through ideas on Discord. I know it's not ideal as not everyone is there, but the real-time nature is important to me. ↩