Run on ubuntu-slim instead of ubuntu-latest (#518)

Author: mscheltienne

.github/workflows/ci.yaml

@@ -13,7 +13,7 @@ on:  # yamllint disable-line rule:truthy
 jobs:
   get_commit_message:
     name: get commit message
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     outputs:
       commit_message: ${{ steps.get_message.outputs.commit_message }}
     steps: