Replaced SetEnvironmentVariable with AppendEnvironmentVariable in ignition.launch.py

[federicociresola]

Description

I discovered that if anyone had set up an environment variable such as IGN_GAZEBO_RESOURCE_PATH and GN_GAZEBO_PLUGIN_PATH, the launcher overwrote them with new path.
To fix this problem, I replaced SetEnvironmentVariable with AppendEnvironmentVariable in this way if the variables already existed, the launcher will append new paths otherwise it will create them.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I discovered this problem when I was trying to spawn a x500 of PX4 in the world with Gazebo Garden (I forked the code and I am making the porting from Ignition to Gazebo garden) but the same problem there is also in the ignition launcher.
Possible way to test it:

• export IGN_GAZEBO_RESOURCE_PATH:=<custom model folder>
• ros2 launch irobot_create_ignition_bringup create3_ignition.launch.py
• spawn the custom model

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[alsora]

Thank you for the contribution!