Prevent building twice when pushing to a PR (#2345)

Created PRs and commit pushes to PRs are all already handled by `pull_request`. Having `push` without limitations causes 2 build runs on every PR commit push.

Also the `push` scenario is only required for pushes (direct or merges) to the master branch.

.github/workflows/default.yml

@@ -1,6 +1,11 @@
 name: Default
 
-on: [push, pull_request, workflow_call]
+on: 
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_call:
 
 jobs: