Slow github actions due to Conda

[Andrew-S-Rosen]

Every time a new commit is made, the GitHub action runs the unit test with a brand new miniconda installation. This turns a series of tests that should take ~1 min into a several minute ordeal. We should cache miniconda and only re-install every N days (or if setup.py or requirements.txt is updated).

The caching is currently broken in the github action.