Make local conformance runs easier

[jku]

From #1682 (review)

Running conformance locally with current sigstore-python git is a little tricky as we don't have any make rules that would setup an environment that

• makes sigstore modules available for import
• allows running pytest so that it actually finds conformance and not our own test suite

There is certainly room for improvement here but I'm not sure what exactly it is

• this all just works in the conformance workflow
• sigstore-conformance itself has a complicated local testing setup with multiple venvs: we could duplicate something like that but that feels too much here...