Use the new ruby-less command to open Ignition GUI

[Diego Ferigo]

This is a reminder that tracks gazebosim/gz-sim#694. If it will land upstream, we should update the current logic used to open the GUI:

gym-ignition/cpp/scenario/gazebo/src/GazeboSimulator.cpp

Lines 295 to 297 in 50caccf

 // Spawn a new process with the GUI 
 pImpl->gazebo.gui = std::make_unique<TinyProcessLib::Process>( 
     "ign gazebo -g -v " + std::to_string(appliedVerbosity));