syz-ci: don't try to create a JobManager when no dashboard specified #12

Job	Run time
gvisor	3m 58s
race_dashboard	1s
dashboard	1s
build	1s
race	1s
aux	4m 47s
old	6m 55s
arch (presubmit_arch_linux)	10m 41s
arch (presubmit_arch_windows)	3m 31s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_darwin)	3m 57s
arch (presubmit_arch_executor)	3m 15s
arch (presubmit_arch_netbsd)	3m 47s
arch (presubmit_arch_freebsd)	6m 16s
	50m 56s

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