Adapt CI trigger to run on PR from fork.

.github/workflows/ci.yml

@@ -1,5 +1,16 @@
 name: CI
-on: [push]
+on:
+  # We would like to trigger for CI for any pull request action -
+  # both from QuantCo's branches as well as forks.
+  pull_request:
+    types: [opened, labeled, unlabeled, synchronize]
+  # In addition to pull requests, we want to run CI for pushes
+  # to the main branch and tags.
+  push:
+    branches:
+      - "main"
+    tags:
+      - "v*"
 
 # Automatically stop old builds on the same branch/PR
 concurrency: