[Merge] ~seb128/livecd-rootfs:canary-snaps-env into livecd-rootfs:ubuntu/master

[Sebastien Bacher]

Thanks for the review, I've pushed an updated version of the change that uses the recommended function for the snapd setup, using it earlier is problematic because it mounts /var/cache/apt over a tmpfs which fails the build on enospace to download the binaries required

the setup function also got changed to use realpath on the directory since that's what mounts is listing and otherwise it's not doing any unmounting

the previous change could probably be split in another commit or merge request if you prefer
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https://code.launchpad.net/~seb128/livecd-rootfs/+git/livecd-rootfs/+merge/393981
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