Since the allocation in the task runner is updated in a separate
goroutine, a race condition may happen where the task is started but the
prestart hooks are skipped because the allocation became terminal.
Checking for a terminal allocation before proceeding with the task start
ensures the task only runs if the prestart hooks are also executed.
Since `shouldShutdown()` only uses terminal allocation status, it
remains `true` after the first transition, so it's safe to check it
again after the prestart hooks as it will never revert to `false`.
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