syz-cluster: select nested vm pool by label #17552

Job	Run time
build	3m 23s
aux	4m 33s
dashboard	2m 41s
arch (presubmit_arch_linux)	10m 33s
arch (presubmit_arch_darwin)	3m 38s
arch (presubmit_arch_executor)	3m 13s
arch (presubmit_arch_freebsd)	6m 15s
arch (presubmit_arch_windows)	3m 31s
race	7m 29s
arch (presubmit_arch_openbsd)	3m 37s
gvisor	3m 46s
old	7m 7s
arch (presubmit_arch_netbsd)	3m 49s
race_dashboard	3m 53s
	1h 7m 28s

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