chore: migrate CI workflows to ubuntu-slim for faster startup times (#4007)

*This type of runner is optimized for automation tasks, issue operations
and short-running jobs. They are not suitable for typical heavyweight
CI/CD builds.*
[[1](https://docs.github.com/en/actions/reference/runners/github-hosted-runners#single-cpu-runners)].

We are not necessarily dependent on faster startups, but that seems to
be becoming the best practice now.

.github/workflows/enforce-pullrequest-rules.yaml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     if: github.event_name == 'pull_request'
     timeout-minutes: 10
     steps: