repart: Always derive fs/luks UUIDs from generated partition UUID

repart: Always derive fs/luks UUIDs from generated partition UUID

When generating verity partitions, we only know the partition UUID
of the verity data and hash partition after doing the verity
formatting. This means we can't use the verity partition UUID as
input for deriving the filesystem/luks UUIDs. Currently, we derive
the filesystem/luks UUID from the null UUID instead, which isn't
ideal. Instead, let's always generate a partition UUID and use it
to derive the fs/luks UUIDs, but only use it as the actual partition
UUID if we're not doing verity for the partition.