Trigger workflow when PR labels are changed. (#1512)

If a PR submitter forgets to add labels before opening the PR, then adds
them later, the workflow does not re-run and the results don't take into
account the label changes.

 All Submissions:

* [X] Have you followed the guidelines in our Contributing document?
* [X] Have you checked to ensure there aren't other open [Pull
Requests](https://github.com/fastly/cli/pulls) for the same
update/change?

Co-authored-by: Anthony Gomez <anthony.gomez@fastly.com>

Author: kpfleming

.github/workflows/pr_test.yml

@@ -4,6 +4,8 @@ on:
       - opened
       - synchronize
       - reopened
+      - labeled
+      - unlabeled
     branches:
       - main
 name: Test