copy snapd system from live layer to target system ourselves if needed

[mwhudson]

I think paired with https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/466202 this would allow us to remove some of the wasted space on the desktop ISO. But I can't realistically test it until there is a newer subiquity in there.