when running `pip install` tests, cache downloads from pypi

when running `pip install` tests, cache downloads from pypi

Since this runs frequently without much change, it makes sense to cache the
downloaded source packages as much as possible.  There are probably better
ways to do this, but this is really easy.

https://stackoverflow.com/questions/4806448/how-do-i-install-from-a-local-cache-with-pip