Improve yamllint ci step

- This package is installed by default in GitHub Actions runners actions/runner-images#1142
- Use github warnings output formatter

.github/workflows/ci.yaml

@@ -50,9 +50,7 @@ jobs:
 
       - name: Lint YAML sources with yamllint
         run: |
-          sudo -H python3 -m pip install --upgrade pip setuptools wheel
-          sudo -H python3 -m pip install --upgrade yamllint
           yamllint --version
           echo "Linting YAML sources with yamllint ..."
-          yamllint --strict .
+          yamllint --strict --format github .
           echo "OK"