[skip ci] Update ci workflow to allow skipping

Author: s1hofmann

.github/workflows/ci.yaml

@@ -11,6 +11,7 @@ on:
 jobs:
   sonar:
     runs-on: ubuntu-latest
+    if: '!contains(github.event.head_commit.message, ''skip ci'')'
     steps:
       - name: Set up Git repository
         uses: actions/checkout@v2