.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
name: CIFuzz
-on: [] # was: [pull_request], but disabled in https://github.com/tailscale/tailscale/pull/7156
+on: [pull_request]
concurrency:
group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
