install to-filesystem unable to replace bootloader.

Installing on a Digital Ocean droplet:

```
tune2fs -O verity /dev/vda1
```

```
podman run --privileged --pid=host --user=root:root \
-v /var/lib/containers:/var/lib/containers \
-v /dev:/dev \
--security-opt label=type:unconfined_t \
-v /:/target \
-v /root/.ssh/authorized_keys:/bootc_authorized_ssh_keys/root \
ghcr.io/jmarrero/ubuntu-bootc:latest \
bootc install to-existing-root \
--acknowledge-destructive \
--skip-fetch-check \
--composefs-backend \
--cleanup \
--root-ssh-authorized-keys /bootc_authorized_ssh_keys/root --insecure --bootloader systemd
```

System fails to boot with:
```
SeaBIOS
Booting from Hard Disk...
error: file '/grub/i386-pc/normal.mod' not found.
Entering rescue mode...
grub rescue>
```

Start by reproducing the Digital Ocean installation using the documented `bootc install to-existing-root` command with `--bootloader systemd`, then inspect the resulting bootloader setup and the reported GRUB rescue failure. Done means the installed droplet boots successfully instead of reporting that `/grub/i386-pc/normal.mod` is missing.

Scritto dal modello di indicizzazione a partire dal testo della issue.