To replicate:
cd to a git checkout of the source code, gnu/system/examples
The hash for my current checkout is 
d7e033b9a153a9e60f52ff64f4eb355c1c3d0a6e which is also the hash for my 
current version of guix.
guix system disk-image -t raw lightweight-desktop.tmpl

lightweight-desktop.tmpl defines an operating-system that uses 
grub-efi-bootloader, and is embedded in the info pages as an example 
operating system. Since it is lightweight, it doesn't take as long to 
build as desktop.tmpl.

Expected results: guix builds the raw image just fine, and that image 
can be copied to an SD card and run on a computer with an efi 
bootloader, just like the counterpart with grub-bootloader. There is 
nothing in the documentation suggesting that this should not work by design.

Actual results: guix fails, see attached log.

-Jesse