bug#36942: Reconfigure broke GRUB

[ison <ison@airmail.cc>]

Continuing this issue from https://issues.guix.gnu.org/issue/36878

guix reconfigure recently broke GRUB for me. When rebooting I get
dropped to a "grub rescue>" shell with an error about
"grub_file_filters" being an unknown symbol.
If I try doing the usual commands to tell GRUB how to boot I just
see the above error repeated, or "unknown command" when I run
things such as insmod or "configfile".

So to fix the problem I can boot to a Guix install disk and do
"guix init" which rebuilds the system using older package
definitions from the disk.
That allows me to get a working system, but if I do another
"guix pull" and reconfigure using the same config file it breaks
GRUB again when I reboot. "guix describe" shows that my latest
attempt was with commit 35600cd.

Here is the bootloader and filesystem sections of my config:
  (bootloader (bootloader-configuration
                (bootloader grub-efi-bootloader)
                (target "/boot/efi")))
  (file-systems (cons* (file-system
                         (device "/dev/sda2")
                         (mount-point "/boot/efi")
                         (type "vfat"))
                       (file-system
                         (device (file-system-label "guixsd-root"))
                         (mount-point "/")
                         (type "ext4"))
                       %base-file-systems))

I should make a note that I usually don't use efi, and I'm not
completely confident it's all set up properly. I do have a
"BIOS boot" partition on /dev/sda1 too, is that even needed with
efi? Although, I have been using this setup, and the above
definitions, for about 6 months now without any bootloader or
filesystem issues. And the same config is being used to fix the
system when GRUB breaks as well as to reconfigure afterward
(causing the breakage). So my guess is some new update is the
culprit.