GitHub Actions: Consider further caching in CI workflow

[lukpueh]

Description of issue or feature request: #1242 added a GitHub Actions based CI workflow, which configures basic pip caching.

Current behavior: Basic pip caching in CI workflow

Expected behavior: Explore and implement further caching to further speed up CI workflow (@jku suggests to try caching the tox directory).

[lukpueh]

Also cache pip install in non-"test" jobs, e.g. "coveralls-fin" introduced in #1252. It would be nice to find a cross-job caching solution.

[jku]

Leaving a note for later: Defining an action of our own in this same repo should allow using the same code across different jobs: Maybe it's not hard to define a "pip-install" action that sets up cache, updates pip and installs/updates the given list of packages.

[jku]

cache pip install in non-"test" jobs, e.g. "coveralls-fin" introduced in #1252

this one might be the only relevant thing left here as

• caching tox is not a good idea: tox is not able to handle some changes in dependencies, so we'd need to build something ourselves... not worth it
• setup-python now handles cache (#1718) so that's not longer ugly