Use Cython cache

[vyasr]

The next release of Cython will include a now publicly-documented caching features for Cythonized outputs. We ought to be able to leverage this for faster builds by using GH's artifact cache or something similar to persist cythonization outputs between runs. It will require some testing, though, and on balance cythonization is a miniscule portion of our total build time so it may not be worth a ton of effort if it turns out to be nontrivial to use.