* run a full livefs test build in a VM running launchpad-buildd, pushing in the new livecd-rootfs package at the relevant point
* attempt to build util-linux in xenial from launchpad-buildd with the old image, verifying that this fails
* build util-linux in xenial from launchpad-buildd with the new image, which now works
OK. This time my test procedure was more careful:
* run a full livefs test build in a VM running launchpad-buildd, pushing in the new livecd-rootfs package at the relevant point
* attempt to build util-linux in xenial from launchpad-buildd with the old image, verifying that this fails
* build util-linux in xenial from launchpad-buildd with the new image, which now works