ci: Expand PR title validation triggers (#30)

Force-pushes from automated PRs (like release-please) do not seem to
trigger any of the normal PR triggers (opened, edited, synchronize).
In fact, even an exhaustive list of types will not work.  So here we
add triggers for reviews, so that the validation will run after
someone approves such a PR.  This is critical since this is a required
status check in most of our repos.  If the workflow doesn't run, the
release PR can't be merged.

.github/workflows/validate-pr-title.yaml

@@ -1,11 +1,18 @@
 name: "Validate PR Title"
 
 on:
+  # NOTE: Force-pushes from automated PRs (like release-please) do not seem to
+  # trigger any of the normal PR triggers (opened, edited, synchronize).  In
+  # fact, even an exhaustive list of types will not work.  So here we add
+  # triggers for reviews, so that the validation will run after someone
+  # approves such a PR.  This is critical since this is a required status check
+  # in most of our repos.  If it doesn't run, the PR can't be merged.
   pull_request_target:
     types:
       - opened
       - edited
       - synchronize
+  pull_request_review:
 
 jobs:
   main: