[ci] Fix missing values during manual workflow dispatch

It seems like the github.event.workflow_run payload is only populated
for non manual runs of the workflow, so this would crash the manual
dispatch

Test plan: https://github.com/facebook/react/actions/runs/11017512571
completes

ghstack-source-id: fce02b17f85ac4762de0561764785882e767c872
Pull Request resolved: https://github.com/facebook/react/pull/31045