[Merge] ~rcj/livecd-rootfs/+git/livecd-rootfs:bugfix/snapd_for_core18 into livecd-rootfs:ubuntu/master

[Robert C Jennings]

>From Iain: I’m on my phone now so can’t check anything, but my memory is that snapd was printing an *explicit* error that the core snap was required, i.e. it was hardcoded for some reason. If that’s fixed these days then no reason I can think of to keep that behaviour. But I’d appreciate someone checking the next desktop build seeds snaps properly.

With that I'll merge this and upload.
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https://code.launchpad.net/~rcj/livecd-rootfs/+git/livecd-rootfs/+merge/382041
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