Use local workflows in PR testing for github-workflows

PRs to github-workflows should use the version of the workflows that are updated by it, not the version that’s currently on `main`.

Author: ahoppen

.github/workflows/pull_request.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   soundness:
     name: Soundness
-    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
+    uses: ./.github/workflows/soundness.yml
     with:
       api_breakage_check_enabled: false
       license_header_check_enabled: false