Speed up GH Actions by caching

[marvinschmitt]

The built script itself is basically instantaneous but the Python setup and package installs take some time.

It's just Python and some simple packages (two at the moment) so we should cache it. Some initial pointers:

• https://adamj.eu/tech/2023/11/02/github-actions-faster-python-virtual-environments/
• https://github.com/actions/cache/blob/main/examples.md#python---pip