grub: Do not check for duplicated devices in mirroredBoots on UEFI (#18625)

When Grub is to be used with UEFI, it is not going to write to any MBR
of any disk. As such, it is safe to use multiple "nodev" device entries
when mirroring the ESP partition to multiple disks.

E.g.:

```
boot.loader.grub = {
  enable = true;
  version = 2;
  zfsSupport = true;
  efiSupport = true;
  mirroredBoots = [
    { devices = [ "nodev" ]; path = "/boot1"; efiSysMountPoint = "/boot1"; }
    { devices = [ "nodev" ]; path = "/boot2"; efiSysMountPoint = "/boot2"; }
    { devices = [ "nodev" ]; path = "/boot3"; efiSysMountPoint = "/boot3"; }
  ];
};

boot.loader.efi.canTouchEfiVariables = true;
```

Fixes #18584