CI: update harmonic workflow: install xacro package

Author: Apolo151

.github/workflows/ros2_gz_harmonic_launch_tests.yaml

@@ -38,6 +38,7 @@ jobs:
                   sudo apt-get update
                   # Install ROS 2 packages
                   sudo apt-get install -y ros-humble-ros-base python3-colcon-common-extensions
+                  sudo apt-get install -y ros-humble-xacro
 
             - name: "Setup Gazebo"
               uses: gazebo-tooling/setup-gazebo@v0.3.0