Run the add_header script on every PR and push except master rucio#4980

The `add_header` GitHub Acitons job is running on every pull request. While this
enforces the usage of the script, it most of the times creates a lot of noise,
since the developer has to change the PR after the test fails. We should have a
more intermediate feedback. Running the job on every push seems reasonable.

.github/workflows/autotest.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   header-lint:
-    if: 'github.event_name == ''pull_request'''
+    if: ${{ github.ref != 'refs/heads/master' }}
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2