fix: Retrieving ros time to work with newer ROS 2 versions

[TannerGilbert]

Currently when building with ROS 2 Humble the following error is encountered:

error: passing ‘std::__shared_ptr_access<const rclcpp::Clock, __gnu_cxx::_S_atomic, false, false>::element_type’ {aka ‘const rclcpp::Clock’} as ‘this’ argument discards qualifiers [-fpermissive]
  320 |       *timestamp = nh->get_clock()->now();

This PR fixes the above error. The code was tested on two Ubuntu 22.04 devices running ROS 2 Humble. I didn't check if this breaks compatibility with older ROS 2 versions but I would find it reasonable if the ROS 2 branch works for the current LTS version.