Consider pinning github runner image version

[melange396]

Instead of using a runner image of ubuntu-latest, use a specific one, such as ubuntu-22.04. Using ubuntu-latest sets us up for occasional updates to its runtime environment (a recent one can be seen in actions/runner-images#10636), and these can have significant changes with deep ramifications.

Pros:

• provides a stable host platform without surprises
• can be tailored to reflect our production environment for consistency
• looks like it wouldve saved us from the problem discussed and addressed in fix unittest.mock import error #1574

Cons:

• we might miss out on "free" performance improvements or security fixes (though i dont think either is really a big concern at this point)
• we will still need to bump the version(s) at some point in the [distant?] future when the image we select inevitably gets deprecated
• itll be a pain to do -- searching for "ubuntu-latest" across all of the org's repositories, shows hits in 88 files!