fix: Ensure CI runs on pull request events (#70)

Author: sea-bass

.github/workflows/ros2_ci.yml

@@ -2,7 +2,9 @@
 # For troubleshooting, see README (https://github.com/ros-industrial/industrial_ci/blob/master/README.rst)
 name: ROS 2 CI
 
-on: [push]
+on:
+  push:
+  pull_request:
 
 jobs:
   industrial_ci: