Make workflow run when a PR is opened, synchronized, or reopened

A user forked cisagov/scan-target-data and created a pull request, but
the required GitHub Action(s) did not run.  This is presumably because
the user does not have Actions enabled in his or her fork.  Ideally,
the required Action(s) would run in cisagov/scan-target-data when a PR
to merge changes back is created.  Based on my reading of this link,
adding the "pull_request" event type should make this happen:
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/events-that-trigger-workflows#pull-request-events-for-forked-repositories

.github/workflows/build.yml

@@ -1,7 +1,7 @@
 ---
 name: build
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build: