system(nix): Forbid import from derivation

Helps prevent issues where we accidentally use an import from derivation
and cause flakes with multiple platforms to fail when running things
like `nix flake check`.

modules/system.nix

@@ -96,6 +96,7 @@ in
         experimental-features = [ "nix-command" "flakes" "repl-flake" ];
         auto-optimise-store = true;
         warn-dirty = false;
+        allow-import-from-derivation = false;
 
         trusted-users = [
           "root"