Alright, I found where the problem was, and it wasn't this branch at all (or any branch for that matter): The update-grub postinst script noops inside containers (as reported by systemd-detect-virt), and therefore you would not see the problem running the build locally, but launchpad now isolates builds inside containers.
+1'ing this, provided publicsuffix is appropriately taken care of (see inline comments).
Alright, I found where the problem was, and it wasn't this branch at all (or any branch for that matter): The update-grub postinst script noops inside containers (as reported by systemd- detect- virt), and therefore you would not see the problem running the build locally, but launchpad now isolates builds inside containers.
+1'ing this, provided publicsuffix is appropriately taken care of (see inline comments).
I will send a branch to fix the build separately.