Don't update [min|max]_exit_code on job array task requeue.

Only do so once the task actually finishes. Otherwise, a requeued task
could set an incorrect max_exit_code even if completed with exit code 0
after re-running again, leading to problems with i.e. other jobs with an
afterok type of dependency on such array relying on the incorrectly set
max_exit_code.

Bug 7552.