Run format check on MacOS; setup-swift seems unreliable on Linux.

This analysis <swift-actions/setup-swift#591 (comment)>
explains why I think it will work.

An alternative would be to run it on Linux in the devcontainer, where Swift is already set up.

.github/workflows/format-check.yml

@@ -21,7 +21,7 @@ on:
 jobs:
   build:
     name: Format check
-    runs-on: ubuntu-latest
+    runs-on: macos-13
     steps:
     - uses: actions/cache@v3
       with: