Resolve #3591249 "Phpstan only twice on ci"

composer run phpstan runs it twice, which is ~15 slower for every time you run it during local development. That adds up during a day, and is just needlessly noisy and painful. (And it's just to generate a CI artifact automatically that you never use locally!)

But then … CI actually runs that twice, resulting in FOUR runs:

Testing instructions

• CI still passes
• composer run phpstan still works exactly the same locally, just … faster.

Closes #3591249