nixosTests.installer: fix non-zfs grub tests
nixosTests.installer: fix non-zfs grub tests

Now that grub2 is built without zfs support when possible, this is a
separate store path which may need to be made available to the installer.

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