Fix unlinking old job out/err files. #8535

Job	Run time
1/4	14m 41s
2/4	11m 59s
3/4	11m 17s
4/4	14m 50s
py-3-latest	21m 35s
flaky	6m 20s
_remote_background_indep_poll	5m 9s
_remote_background_indep_tcp	6m 46s
macos 1/5	17m 33s
macos 2/5	19m 46s
codecov	10s
	2h 10m 6s

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