feat: specify submodule branch and improve goal success handling

- Added `branch = main` to the `env_manager` submodule in `.gitmodules`.
- Set the `success` flag in `m_current_result` on goal success in `mtp_jtc.cpp`.

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "env_manager"]
 	path = env_manager
 	url = https://gitlab.com/solid-sinusoid/env_manager.git
+	branch = main

rbs_skill_servers/src/mtp_jtc.cpp

@@ -108,6 +108,7 @@ protected:
                    &wrapped_result) {
           if (wrapped_result.code == rclcpp_action::ResultCode::SUCCEEDED) {
             RCLCPP_INFO(this->get_logger(), "Goal succeeded");
+            m_current_result->success = true;
             m_current_goal_handle->succeed(m_current_result);
           } else {
             RCLCPP_ERROR(this->get_logger(), "Goal failed");