We enforce exactly one plugin supervisor loop by checking whether
`running` is set and returning early. This works but is fairly
subtle. It can briefly result in two goroutines where one quickly
exits before doing any work. Clarify the intent by using
`sync.Once`. The goroutine we've spawned only exits when the entire
task runner is being torn down, and not when the task driver restarts
the workload, so it should never be re-run.
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