Allow user to use (nix) sandbox

configuration.nix

@@ -55,6 +55,7 @@ in {
 
   system.stateVersion = "19.09";
 
+  nix.trustedUsers = [ "user" ];
   nix.optimise.automatic = true;
   nix.gc.automatic = true;
   nix.gc.options = "--delete-older-than 7d";