[live-usb] Clean up config

Also switched from trying to override the `nixos` user to instead just
installing home-manager _into_ the `nixos` user. It felt a bit like
fighting the tide otherwise.

flake.nix

@@ -80,7 +80,11 @@
       modules = baseModules ++ [
         ./machines/live-usb/configuration.nix
         {
-          home-manager.users.johno = import ./home/home.nix;
+          home-manager.users.nixos = { ... }: {
+            imports = [ ./home/home.nix ];
+            home.username = nixpkgs.lib.mkForce "nixos";
+            home.homeDirectory = nixpkgs.lib.mkForce "/home/nixos";
+          };
           home-manager.extraSpecialArgs = { inherit system; };
         }
       ];