nixos/nix: move trusted-user to config

This avoids having to define root when trying to add another user as
trusted user, similar to the other options.

nixos/modules/config/nix.nix

@@ -302,7 +302,6 @@ in
 
             trusted-users = mkOption {
               type = types.listOf types.str;
-              default = [ "root" ];
               example = [ "root" "alice" "@wheel" ];
               description = ''
                 A list of names of users that have additional rights when
@@ -376,6 +375,7 @@ in
     environment.etc."nix/nix.conf".source = nixConf;
     nix.settings = {
       trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
+      trusted-users = [ "root" ];
       substituters = mkAfter [ "https://cache.nixos.org/" ];
       system-features = mkDefault (
         [ "nixos-test" "benchmark" "big-parallel" "kvm" ] ++

nixos/modules/profiles/installation-device.nix

@@ -125,7 +125,7 @@ with lib;
     '';
 
     # allow nix-copy to live system
-    nix.settings.trusted-users = [ "root" "nixos" ];
+    nix.settings.trusted-users = [ "nixos" ];
 
     # Install less voices for speechd to save some space
     services.speechd.package = pkgs.speechd.override {

nixos/modules/profiles/macos-builder.nix

@@ -123,7 +123,7 @@ in
 
       max-free = cfg.max-free;
 
-      trusted-users = [ "root" user ];
+      trusted-users = [ user ];
     };
 
     services = {