iso-image: Fix grub file load location
iso-image: Fix grub file load location

With U-Boot UEFI, (hd0) is not the USB drive, it is (cd0).

Though, it turns out we never needed to prefix the path!

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs