Kill tasks during job prep #8626

Job	Run time
1/4	13m 44s
2/4	14m 22s
3/4	13m 2s
4/4	14m 35s
py-3-latest	19m 55s
flaky	6m 12s
_remote_background_indep_poll	4m 55s
_remote_background_indep_tcp	7m 2s
macos 1/5	22m 46s
macos 2/5	17m 25s
codecov	11s
	2h 14m 9s

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