Cache poetry install + unify Python/Poetry setup for lint and test jobs. (#9625)

With this PR:
- All lint and test jobs use the exact same Python + Poetry installation
approach, instead of lints doing it one way and tests doing it another
way.
- The Poetry installation itself is cached, which saves ~15s per run.
- We no longer pass shell commands as workflow arguments to a workflow
that just runs them in a shell. This makes our actions more resilient to
shell code injection.

If y'all like this approach, I can modify the scheduled tests workflow
and the release workflow to use this too.