Having an empty `on` spec results in the job still running, but it
immediately fails with a "No jobs were run" message.
Go back to the original `on: [pull_request]` spec, and disable the
workflow in the GitHub UI instead.
This reverts commit f7b3156f1635d539bc59e5015661b235f80f2b90.
Signed-off-by: Mihai Parparita <mihai@tailscale.com>