ci: fix UX for prettier failures

(cherry picked from commit bb8f23e, PR #9926)

Author: ErichDonGubler

.github/workflows/ci.yml

@@ -816,8 +816,10 @@ jobs:
           rustup override set ${{ env.REPO_MSRV }}
           cargo -V
 
+      # NOTE: Keep this above the others, because this will fail if there are _any_ changes to
+      # files, even ones this step doesn't modify.
       - name: "Format `**/*.{md,js,html,yml}` files"
-        run: npx --yes prettier@3.8.1 --check .
+        run: "npx --yes prettier@3.8.1 --check --write . && git diff --exit-code"
 
       - name: Run `cargo fmt`
         run: |