CI: Don't run CI on non-master pushs

To avoid running twice, when opening a PR.

Author: larskanis

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: CI
 on:
   push:
+    branches:
+      - master
+    tags:
+      - "*.*.*"
   pull_request:
   schedule:
   - cron:  '30 20 * * *'