[Merge] ~enr0n/ubuntu-release-upgrader:ubuntu/main into ubuntu-release-upgrader:ubuntu/main

[Nick Rosbrook]

Alright, yeah I was missing something with the Gio.Settings approach, so shelling out to gsettings seems the way to go. I did this with `systemd-run --user -M $user at .host ...` to simplify the permissions/user env. That way, there is no need to change our running env or EUID -- just need to get the original UID via SUDO_UID or PKEXEC_UID.
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https://code.launchpad.net/~enr0n/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/452304
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