Guix bios installation: Grub error: unknown filesystem

[Franz Geffke]

I'm having trouble installing guix in qemu, using a "fresh" guix ISO.

```
building 
/gnu/store/byjlc85abyjc3fjj9z982677skmda7ib-module-import-compiled.drv...
building 
/gnu/store/psw8xn9qpsjjnrqmjrfv0v3jj9fphq5m-module-import-compiled.drv...
building 
/gnu/store/a1zcrrcdwhb4wb2g4r0ph8mqclq7f5xn-install-bootloader.scm.drv...
guix system: error: 
'/gnu/store/li1ic17hz460vp1sg0cx2dwgw8q7i8pj-grub-2.06/sbin/grub-install 
--no-floppy --target=i386-pc --boot-directory /mnt/boot /dev/sda' exited with 
status 1; output follows:

  Installing for i386-pc platform.
  /gnu/store/li1ic17hz460vp1sg0cx2dwgw8q7i8pj-grub-2.06/sbin/grub-install: 
error: unknown filesystem.
```

Here's what I've tried so far:
1. The ISO from 2022 
https://ftpmirror.gnu.org/gnu/guix/guix-system-install-1.4.0.x86_64-linux.iso: 
Success
2. Generated a new ISO today: Failure

These are the channels, on the booted ISO:

```
guix describe
  guix 65e8472
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 65e8472a4b6fc6f66871ba0dad518b7d4c63595e
```

Steps I used to install (1) and (2):

```
parted -s /dev/sda -- mklabel msdos mkpart primary ext4 1MiB 100%
parted /dev/sda set 1 boot on
mkfs.ext4 -L my-root /dev/sda1
mount LABEL=my-root /mnt
cp /etc/configuration/lightweight-desktop.scm /mnt/etc/config.scm 
# adjust disk, bootloader
herd start cow-store /mnt
guix system init /mnt/etc/config.scm /mnt
```

Findings:

I didn't really dig too deeply yet; Only noticed that this command produces a 
different result, depending on whether the install succeeds, or not `grub-probe 
--target=fs --device /dev/sda`

- Success: `ext2` 
- Failure: `grub-probe: error: unknown filesystem.`

I also tried using GPT instead of MBR, but it makes no difference.