[CI] Ensure the PHPStan job is executed within the /build directory

This is impossible to test - at least not without creating an entire fork of Drupal core and its scheduled pipelines to run against gitlab, but the changes are essentially reverting part of the previous issue so I think we should go ahead here - if pipelines are green we haven't made anything worse, we should properly check that phpstan baseline caching works after commit.

Per #4 pipelines are green and my limited devOps eyes doesn't see anything glaring.

This issue rings a bell with an earlier issue I can't recall off the top of my head, but from memory we were doing extra work that shouldn't have been necessary, but was because of inconsistent locations. Left a comment on the MR asking about the sed calls.

#7 that’s 📌 [CI] Stop post editing regenerate PHPStan baseline Active . It’s blocked on a PHPStan issue upstream.