Investigate adding package caching to GitHub Actions CI

For the test jobs in the GitHub Actions CI (see #1287), we're not currently caching the `pip`-installed packages used. We could do that, at the expense of some extra complexity in the job descriptions.

See https://github.com/actions/cache and in particular the `pip`-specific examples and suggestions here: https://github.com/actions/cache/blob/main/examples.md#python---pip

This is low priority, since the GitHub actions jobs are already way faster than our existing CI. It may not be a good tradeoff at all if it adds too much complexity to the job configuration.