syz-cluster: adjust the limits on parallel processing #17533

Job	Run time
build	1s
aux	16s
old	14s
race_dashboard	16s
dashboard	1s
arch (presubmit_arch_linux)	15s
gvisor	15s
arch (presubmit_arch_freebsd)	15s
arch (presubmit_arch_windows)	17s
arch (presubmit_arch_openbsd)	16s
race	16s
arch (presubmit_arch_executor)	15s
arch (presubmit_arch_darwin)	16s
arch (presubmit_arch_netbsd)	15s
	3m 8s

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