Run GitHub CI on Pull Requests to main

Summary:
We currently run the main CI job on pushes only. This means we don't automatically run CI on pull requests from third-party users.
This is not ideal and can create delays for trivial issues such as bad formatting (see [example](#114)).

Reviewed By: TD5

Differential Revision: D80930880

fbshipit-source-id: d3f5a460fdf92a3e7a1842e07a0b97e098a7c197

Author: robertoaloi

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: ELP CI
 on:
   push: {}
+  pull_request:
+    branches:
+      - main
   release:
     types: [published]