[Merge] ~toabctl/livecd-rootfs:jammy-fix-snap-validate-seed into livecd-rootfs:ubuntu/jammy

[Steve Langasek]

maybe it's snapd itself that is writing to this tree, when bind-mounted under /sys?  In which case a solution here is, instead of backup+restore, on each call copying the tree from /usr to a sanely-writable location (like we already do in the top of live-build/auto/config for various bits) and at the end to remove it again.
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https://code.launchpad.net/~toabctl/livecd-rootfs/+git/livecd-rootfs-1/+merge/445180
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