fix: improve handling of zfs tag for images (#62) #277

Job	Run time
Get CoreOS versions	4s
Get Build Info	1s
Build CoreOS ZFS (fedora-coreos-zfs, stable)	3m 8s
Build CoreOS ZFS (fedora-coreos-zfs, testing)	2m 52s
Build uCore (ucore, stable, -zfs)	3m 20s
Build uCore (ucore, stable)	3m 18s
Build uCore (ucore, testing, -zfs)	3m 19s
Build uCore (ucore, testing)	3m 23s
Build HCI (ucore, stable, -zfs)	3m 14s
Build HCI (ucore, stable)	3m 0s
Build HCI (ucore, testing, -zfs)	3m 7s
Build HCI (ucore, testing)	3m 31s
	32m 17s

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