guix sytstem image --image-type=qcow2 --image-size=<size> ... builds a raw disk-image of <size> first before converting it to qcow2. This prevents us from specifying size altogether, since it'll exceed the disk space of host while building.

This issue needs to be fixed in guix first. We can remove several steps from images/guix/genimg for resizing said minimal qcow2 image afterwards.