Pin version of ruamel.yaml to avoid checks breaking

Author: henrymercer

.github/workflows/pr-checks.yml

@@ -52,7 +52,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install ruamel.yaml
+          pip install ruamel.yaml@0.17.31
 
       # Ensure the generated PR check workflows are up to date.
       - name: Verify PR checks up to date