FWIW, I've run into this, too, with MAAS 1.7 and UEFI. The first error I noticed was that /usr/lib/grub/modinfo.sh doesn't exist. It DOES exist one further directory down on the mounted filesystem (/tmp/tmpqRoeEj/target/usr/lib/grub/x86_64-efi/modinfo.sh). I'm not sure if the installer is trying to read from the installed system or from the tempfs at this point, but adding that one directory element to the script that calls modinfo.sh might be all that's needed to fix the bug.
FWIW, I've run into this, too, with MAAS 1.7 and UEFI. The first error I noticed was that /usr/lib/ grub/modinfo. sh doesn't exist. It DOES exist one further directory down on the mounted filesystem (/tmp/tmpqRoeEj /target/ usr/lib/ grub/x86_ 64-efi/ modinfo. sh). I'm not sure if the installer is trying to read from the installed system or from the tempfs at this point, but adding that one directory element to the script that calls modinfo.sh might be all that's needed to fix the bug.