Use versionned ubuntu image for the job

Author: vstinner

.github/workflows/build.yml

@@ -120,7 +120,7 @@ jobs:
 
   check_generated_files:
     name: 'Check if generated files are up to date'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     timeout-minutes: 60
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'