.github/workflows: re-enable CIFuzz job

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 f7b3156f16.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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