Run GitHub Actions tests on PRs as well

When initially creating the GitHub Actions
workflow, it was obviously forgotten to make
it also run on pull requests, not only when
changes are pushed to branches in the
original repository (not a fork).

.github/workflows/verify.yml

@@ -1,6 +1,6 @@
 name: Verify changes
 
-on: [push]
+on: [push, pull_request]
 
 jobs:       
   build: