executor: do output area cleanup before each execution #17439

Job	Run time
build	3m 56s
race	7m 15s
dashboard	2m 39s
aux	4m 3s
old	6m 57s
gvisor	3m 49s
race_dashboard	3m 21s
arch (presubmit_arch_freebsd)	6m 20s
arch (presubmit_arch_linux)	10m 41s
arch (presubmit_arch_executor)	3m 19s
arch (presubmit_arch_netbsd)	3m 55s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_openbsd)	3m 41s
arch (presubmit_arch_darwin)	3m 56s
	1h 7m 28s

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