Enable CI workflow on PRs

The `push` event only triggers for commits to this repo and will not run
on commits in a PR from a forked repo. This becomes an issue since CI is
required to merge a PR.

Author: shanesmith

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 name: CI
 
-on: push
+on: [push, pull_request]
 
 permissions:
   contents: read