.github/workflows: Avoid duplicate branch builds

Run CI and package builds for pull requests and for pushes to master.
Avoid running both event types for each same-repository PR update.

Keep manual package workflow runs available.

.github/workflows/ci.yml

@@ -1,6 +1,12 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 
 env:
   WERROR: 'y'