Cancel running GH action on update of PR branch

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

.github/workflows/main.yml

@@ -12,6 +12,10 @@ on:
       - develop
       - /^release\/.*$/
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   Docs:
     runs-on: ubuntu-20.04