tools/create-image.sh: add option to specify output prefix #18452

Job	Run time
race_dashboard	3m 11s
build	5m 21s
dashboard	1m 55s
gvisor	3m 58s
aux	3m 55s
old	7m 4s
race	8m 53s
arch (presubmit_arch_openbsd)	3m 54s
arch (presubmit_arch_darwin)	3m 54s
arch (presubmit_arch_freebsd)	6m 26s
arch (presubmit_arch_netbsd)	3m 56s
arch (presubmit_arch_executor)	3m 12s
arch (presubmit_arch_windows)	3m 34s
arch (presubmit_arch_linux)	9m 52s
	1h 9m 5s

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