`pypackage`: cache the `.tox` directory on GHA

The `pypackage` cookiecutter should cache the `.tox` directory on GitHub Actions in the same way as the `pyramid-app` one does. This might speed up CI and [tox-recreate](https://github.com/hypothesis/tox-recreate) should ensure that the venvs get recreated if `setup.cfg` changes (and tox itself will recreate the venvs if `tox.ini` changes).

For packages, which use unpinned dependencies, we actually _want_ the venvs on CI to be re-created over time even if `tox.ini` and `setup.cfg` don't change so that we test against new versions of our dependencies as they get released. Caching the `.tox` dir would defeat that. I wonder if we could disable caching just for the scheduled nightly run of `ci.yml`?