Add support for pip caching.

This isn't quite working yet. The tests are a little funky, and I need to add CLI flag support

* [x] envvar support
* [x] positive test
* [ ] negative test
* [ ] confidence in tests
* [ ] cli flag support
For that matter, why don't we just merge those 2 helper methods, install_from_path and install_from_string, into ServerTestCase and eliminate a class? Those methods pretty much assume a local server anyway.