[Merge] livecd-rootfs:xnox/mksquashfs-zstd into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

I checked a bit more carefully: the desktop squashfses with this change are 4.3% or about 112 MiB bigger. That's not a lot but it's also not nothing. I think we'd want to demonstrate a noticeable performance hit to justify that (and that's not something I have yet got around to seriously trying)
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/449608
Your team Ubuntu Core Development Team is requested to review the proposed merge of livecd-rootfs:xnox/mksquashfs-zstd into livecd-rootfs:ubuntu/master.