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
Whoops! Don't tread on this. Maybe you could respell it with adjacent string literals, like `"...stripped" "   "`, so it doesn't get killed in the future.