[Merge] ~tobijk/livecd-rootfs/+git/livecd-rootfs:feature/snap-tool-prefer-stable-for-core into livecd-rootfs:ubuntu/master

[Łukasz Zemczak]

...ok, apparently snap prepare-image won't do what we want, so I guess we should just proceed with the workaround. Thank you!
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https://code.launchpad.net/~tobijk/livecd-rootfs/+git/livecd-rootfs/+merge/364800
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