tools/syz-imagegen: accept filesystem descriptions as input #17711

Job	Run time
race	7m 23s
aux	4m 43s
race_dashboard	3m 39s
build	2m 16s
old	7m 4s
gvisor	3m 46s
arch (presubmit_arch_linux)	10m 16s
arch (presubmit_arch_openbsd)	3m 49s
arch (presubmit_arch_freebsd)	6m 31s
arch (presubmit_arch_darwin)	3m 32s
arch (presubmit_arch_windows)	3m 35s
arch (presubmit_arch_executor)	3m 20s
dashboard	2m 42s
arch (presubmit_arch_netbsd)	3m 56s
	1h 6m 32s

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