ci: Fix path filter in reporter workflow (#4654)

Author: rschristian

.github/workflows/pr-reporter.yml

@@ -22,7 +22,11 @@ jobs:
       - uses: dorny/paths-filter@v3
         id: filter
         with:
-          ref: ${{ github.event.workflow_run.head_sha }}
+          # As this Workflow is triggered by a `workflow_run` event, the filter action
+          # can't automatically assume we're working with PR data. As such, we need to
+          # wire it up manually with a base (merge target) and ref (source branch).
+          base: ${{ github.event.workflow_run.pull_requests[0].base.sha }}
+          ref: ${{ github.event.workflow_run.pull_requests[0].head.sha }}
           # Should be kept in sync with the filter in the CI workflow
           filters: |
             jsChanged: '**/src/**/*.js'