nix: make OVMF environment variables optional

Since rust-osdev#798 they are not required
anymore to enable "cargo xtask run"

Author: phip1611

shell.nix

@@ -12,8 +12,6 @@ pkgs.mkShell rec {
   ];
 
   # Set ENV vars.
-  # These are automatically the right files for the current CPU (if available).
-  # https://github.com/NixOS/nixpkgs/blob/nixos-22.11/pkgs/applications/virtualization/OVMF/default.nix#L80
-  OVMF_CODE="${pkgs.OVMF.firmware}";
-  OVMF_VARS="${pkgs.OVMF.variables}";
+  # OVMF_CODE="${pkgs.OVMF.firmware}";
+  # OVMF_VARS="${pkgs.OVMF.variables}";
 }