Check all packages in CI

In commit 26fd110 I tried adding the new --workspace flag to cargo check and cargo test in order to check all crates, but then the CI fails when trying to build pineappl_py, which doesn't find a Python 3 interpreter. I thought this might be a simple missing setup-python action, but this doesn't change anything.

wait a minute: you run in a container there, no? so you need python inside there, no?

Right 😄, that makes sense! 👍