Run GitHub actions also on pull requests

The push trigger didn't seem to work for PRs based in outside repos. So
limit the push trigger to master and next and enable a PR trigger.

.github/workflows/build.yml

 name: Test Build
 on:
   push:
+    branches:
+      - master
+      - next
+  pull_request:
   schedule:
     - cron: '0 0 * * 0' # Every Sunday at 00:00
 jobs: