pkg/instance: fix sandbox_arg passing #17541

Job	Run time
race_dashboard	3m 5s
race	9m 36s
aux	4m 55s
build	3m 19s
arch (presubmit_arch_darwin)	3m 40s
arch (presubmit_arch_netbsd)	5m 47s
arch (presubmit_arch_windows)	3m 33s
arch (presubmit_arch_openbsd)	3m 41s
arch (presubmit_arch_executor)	3m 18s
arch (presubmit_arch_freebsd)	6m 27s
dashboard	2m 36s
old	6m 54s
arch (presubmit_arch_linux)	10m 29s
gvisor	3m 55s
	1h 11m 15s

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