syz-cluster: update README #18411

Job	Run time
dashboard	2m 1s
old	7m 1s
race	7m 16s
gvisor	4m 3s
aux	4m 40s
build	6m 16s
race_dashboard	3m 8s
arch (presubmit_arch_linux)	10m 25s
arch (presubmit_arch_openbsd)	3m 55s
arch (presubmit_arch_darwin)	3m 46s
arch (presubmit_arch_freebsd)	6m 17s
arch (presubmit_arch_executor)	3m 6s
arch (presubmit_arch_netbsd)	3m 54s
arch (presubmit_arch_windows)	3m 32s
	1h 9m 20s

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