chore: run GitHub CI action on more pull request activity

This corresponds to a similar commit on the C++ jsonnet repo:
google/jsonnet@a02a615

The is intended to make it easier for me to trigger running CI on
existing pull requests.

.github/workflows/ci.yml

@@ -1,6 +1,7 @@
 name: ci
 on:
   pull_request:
+    types: [opened, reopened, synchronize, ready_for_review, review_requested]
     branches:
       - master
   push: