[Merge] ~toabctl/livecd-rootfs:master-replace-apt-key-lp#1948929 into livecd-rootfs:ubuntu/master

[Robert C Jennings]

Review: Approve

Looks great, +1 from me.  Have you tested with this change?  And what options does a reviewer need to use to exercise this code path?
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https://code.launchpad.net/~toabctl/livecd-rootfs/+git/livecd-rootfs-1/+merge/410866
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