Comment 5 for bug 1648278

I tested again, step (5) needs to be run in the /target, so here's an oneliner for both steps (4) and (5):

sudo sed 's/-signed"/"/' -i /usr/share/grub-installer/grub-installer; while [ ! -f /target/bin/efibootmgr ]; do sleep 1; done; sudo ln -sf true /target/bin/efibootmgr

After entering that in a terminal, ubiquity can be started and it finishes successfully.