Kill tasks during job prep #8656

Job	Run time
1/4	12m 55s
2/4	13m 42s
3/4	12m 14s
4/4	16m 40s
py-3-latest	17m 48s
flaky	6m 33s
_remote_background_indep_poll	5m 23s
_remote_background_indep_tcp	7m 16s
macos 1/5	25m 37s
macos 2/5	19m 51s
codecov	7s
	2h 18m 6s

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