[ci] cache tox environments

To speed up our CI runs, it would be best to utilize [GitHub actions' cache feature](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows)

An example commit from a different project using this feature for reference:
https://github.com/postlund/pyatv/commit/055c5d2caeafd3998bc1345add8ef881dc1d39d0