CI: Evaluate setup-msys2 version 2.3.1, potentially update to it
Now there's v2.3.1 of setup-msys2 available. Evaluate if it would work better (without similar compatibility hack relative to our 210515 snapshot) than 2.3.0 currently in use. If yes, update.
Attached patch that I test this with.
It passed initial testing, so now truly considered for inclusion.