Remove GH action trigger on pull requests (#1438)

Currently, creating a pr will trigger to run the gh action twice. Remove the trigger `pull_request` to run only once.

.github/workflows/main.yml

@@ -1,5 +1,5 @@
 name: CI
-on: [ push, pull_request ]
+on: [ push ]
 jobs:
   run-unit-tests:
     name: Run unit tests