fix: improve handling of zfs tag for images #275

Job	Run time
Get CoreOS versions	3s
Get Build Info	2s
Build CoreOS ZFS (fedora-coreos-zfs, stable, pr-62-)	2m 46s
Build CoreOS ZFS (fedora-coreos-zfs, testing, pr-62-)	3m 29s
Build uCore (ucore, stable, false, pr-62-)	3m 37s
Build uCore (ucore, stable, true, pr-62-)	3m 37s
Build uCore (ucore, testing, false, pr-62-)	3m 6s
Build uCore (ucore, testing, true, pr-62-)	3m 3s
Build HCI (ucore, stable, -zfs, pr-62-)	3m 35s
Build HCI (ucore, stable, pr-62-)	2m 54s
Build HCI (ucore, testing, -zfs, pr-62-)	3m 44s
Build HCI (ucore, testing, pr-62-)	3m 47s
	33m 43s

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