executor: favor MAP_FIXED_NOREPLACE over MAP_FIXED #15433

Job	Run time
build	4m 22s
aux	4m 30s
dashboard	2m 48s
old	6m 4s
gvisor	3m 16s
arch (presubmit_arch_linux)	9m 21s
race	9m 29s
arch (presubmit_arch_freebsd)	5m 35s
race_dashboard	3m 27s
arch (presubmit_arch_netbsd)	3m 44s
arch (presubmit_arch_openbsd)	3m 20s
arch (presubmit_arch_darwin)	3m 31s
arch (presubmit_arch_windows)	3m 21s
arch (presubmit_arch_executor)	3m 5s
	1h 5m 53s

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