Guixsd vs Heads - How to make it work together?

Nobody has claimed this yet.

@tlaurion @jfrederickson @daym

Installation complete without any issues. But `heads` can't boot/sign separate `/boot` partition.
I use simple unencrypted `/boot` and encrypted `/` as LUKS.
Here is an `install script` and `config.scm`. Any ideas why?

```
(bootloader (bootloader-configuration
(bootloader grub-bootloader)
(target "/dev/sda")))

(initrd-modules (cons* "i915" %base-initrd-modules))

(mapped-devices
(list (mapped-device
(source (uuid ""))
(target "box")
(type luks-device-mapping))))

(file-systems (append (list (file-system
(device "/dev/mapper/box")
(mount-point "/")
(type "btrfs")
(dependencies mapped-devices))
(file-system
(device "/dev/sda1")
(mount-point "/boot")
(type "ext4")))
%base-file-systems))
```

```
rfkill unblock all
cat << EOF > /etc/wpa_supplicant.conf
network={
ssid=""
psk=""
}
EOF
herd stop wpa-supplicant
wpa_supplicant -B -i wlp2s0 -c /etc/wpa_supplicant.conf
dhclient -v wlp2s0

modprobe dm_mod
echo -e "o\nn\np\n1\n\n+128M\nn\np\n2\n\n\nt\n2\n8e\nw" | fdisk /dev/sda
cfdisk /dev/sda (/dev/sda1 - flag bootable)
cryptsetup --cipher aes-xts-plain64 --key-size 512 --hash sha512 --iter-time 5000 --use-random luksFormat /dev/sda2
cryptsetup luksOpen /dev/sda2 box
mkfs.btrfs -L root /dev/mapper/box
mount LABEL=root /mnt
mkdir -pv /mnt/boot
mkfs.ext4 -L boot /dev/sda1
mount LABEL=boot /mnt/boot/

herd start cow-store /mnt
mkdir /mnt/etc

blkid -s UUID -o value /dev/sda2 >> /mnt/etc/config.scm

nano /mnt/etc/config.scm
guix system init /mnt/etc/config.scm /mnt
reboot
```

Start with the supplied install script and /etc/config.scm, focusing on the GRUB bootloader, encrypted root mapping, and separate /boot filesystem. Reproduce the installation and boot/signing failure in a comparable Heads environment, then identify the required compatible configuration or document why this setup cannot work.

Written by the indexing model from the issue text.