syz-cluster: adjust the limits on parallel processing #17527

Job	Run time
aux	4m 33s
arch (presubmit_arch_darwin)	3m 45s
arch (presubmit_arch_netbsd)	3m 43s
arch (presubmit_arch_freebsd)	6m 24s
race_dashboard	2m 50s
arch (presubmit_arch_openbsd)	3m 49s
dashboard	2m 31s
arch (presubmit_arch_linux)	10m 29s
build	3m 13s
arch (presubmit_arch_windows)	3m 47s
old	7m 13s
gvisor	3m 54s
arch (presubmit_arch_executor)	3m 16s
race	7m 13s
	1h 6m 40s

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