Avoiding double builds in GitHub Actions

I was recently setting up some CI on GitHub Actions and ran across double builds for pull requests. The official documentation is still a bit lacking and it's a problem because it burns 2x the credits.

In the end, I made this diff:

-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    branches:
+      - master

This runs the GitHub Action on all PRs and the master branch. If you've got staging or dev branches, you'll want to add them too.