Fix eval in flakes when aliases are disabled

[SuperSandro2000]

Fixes

       error: attribute 'hostPlatform' missing

       at /nix/store/8hqjsxybh8fvkij104linyyp3gi3c560-source/flake.nix:82:27:

           81|         let
           82|           rustTarget = "${pkgs.hostPlatform.qemuArch}-unknown-uefi";
             |                           ^
           83|           pkgs = import nixpkgs {
       Did you mean rustPlatform?

regressed in #31