refactor(nixpkgs): drop the repo-wide pkgs-x64 package set

Every remaining pkgs-x64 consumer runs only on x86_64-linux hosts, where the
native pkgs already carries the same allowUnfree config
(modules/nixos/base/nix.nix) and overlays (modules/base/overlays.nix), so
replace the references with pkgs and drop the extra nixpkgs instantiation
(which also removes an eval on aarch64-darwin).

media.nix keeps the native Zoom app on x86_64 hosts only, since zoom-us has
no aarch64 Linux build.

outputs/default.nix

@@ -56,15 +56,6 @@ let
           cudaCapabilities = [ "8.9" ];
         };
       };
-
-      pkgs-x64 = import nixpkgs {
-        system = "x86_64-linux";
-
-        # To use chrome, we need to allow the installation of non-free software
-        config.allowUnfree = true;
-
-        overlays = import ../overlays args;
-      };
     };
 
   # This is the args for all the haumea modules in this folder.