Fix the flag name

jupyterlab_server/handlers.py

@@ -83,7 +83,7 @@ def get_page_config(self) -> dict[str, Any]:
         server_root = server_root.replace(os.sep, "/")
         base_url = self.settings.get("base_url")
         server_app = self.settings.get("serverapp")
-        allow_setup_custom_env_variables = server_app.allow_custom_env_variables
+        allow_setup_custom_env_variables = server_app.allow_setup_custom_env_variables
 
         # Remove the trailing slash for compatibility with html-webpack-plugin.
         full_static_url = self.static_url_prefix.rstrip("/")