Fix nushell template

I'm dum :(

data/env.nu.j2

@@ -9,7 +9,7 @@ def _nix_your_shell (command: string, args: list<string>) {
     {%- else %}
     let args = ["--"] ++ $args
     {%- endif %}
-    run-external {{ executable }} $command ...$args
+    run-external {{ executable }} nu $command ...$args
   } else {
     run-external $command ...$args
   }