[Merge] ~philroche/livecd-rootfs/+git/livecd-rootfs:bugfix/ubuntu-cpc-filelist-sort-LP-2033677 into livecd-rootfs:ubuntu/master

[Philip Roche]

Tested with [ubuntu-old-fashioned](https://github.com/ubuntu-bartenders/ubuntu-old-fashioned/) to produce a squashfs image resulting in sorted filelist https://pastebin.ubuntu.com/p/vZXYN5jMwh/ 
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https://code.launchpad.net/~philroche/livecd-rootfs/+git/livecd-rootfs/+merge/450372
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~philroche/livecd-rootfs/+git/livecd-rootfs:bugfix/ubuntu-cpc-filelist-sort-LP-2033677 into livecd-rootfs:ubuntu/master.