Run github actions on pull request

When this was in my personal namespace and I was the only one
contributing `push` was effectively the same as `pull_request`, but now
it turns out the GitHub actions haven't been triggering! Alas.

Author: pganssle

.github/workflows/tests.yml

@@ -1,6 +1,6 @@
 name: tzdata contents
 
-on: [push]
+on: [pull_request]
 
 jobs:
   tests: