Kill tasks during job prep #8620

Job	Run time
1/4	12m 26s
2/4	13m 56s
3/4	12m 45s
4/4	14m 48s
py-3-latest	19m 15s
flaky	6m 30s
_remote_background_indep_poll	5m 12s
_remote_background_indep_tcp	7m 2s
macos 1/5	24m 37s
macos 2/5	21m 29s
codecov	7s
	2h 18m 7s

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