CI checks aren't enforcing Prettier

[JoshuaKGoldberg]

Overview

Sibling to #1100: this was never checked because the CircleCI config doesn't enforce that prettier --list-different has no entries.

Actual Behavior

You can git commit --no-verify and git push non-Prettier-ok code.

Expected Behavior

🙅

[JoshuaKGoldberg]

Wait, #1127 didn't enforce this in CI.