Update autogenerated notice to refer to specific ruamel.yaml version

Author: henrymercer

.github/workflows/__analyze-ref-input.yml

@@ -52,6 +52,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
+          # When updating this, update the autogenerated code header in `sync.py` too.
           pip install [email protected]
 
       # Ensure the generated PR check workflows are up to date.

.github/workflows/__autobuild-action.yml

@@ -21,10 +21,11 @@
     "nightly-latest"
 ]
 
-
+# When updating the ruamel.yaml version here, update the PR check in
+# `.github/workflows/pr-checks.yml` too.
 header = """# Warning: This file is generated automatically, and should not be modified.
 # Instead, please modify the template in the pr-checks directory and run:
-#     (cd pr-checks; pip install ruamel.yaml && python3 sync.py)
+#     (cd pr-checks; pip install ruamel.yaml@0.17.31 && python3 sync.py)
 # to regenerate this file.
 
 """