DISPLAY environment variable race condition when checking GPU support

[Crola1702]

Reference build: https://build.osrfoundation.org/job/_test_ignition_gazebo-ci-gz-sim7-focal-amd64/1/ (Test job)

Log output:

++++ echo 'GPU support needed by the script but DISPLAY var is empty'
GPU support needed by the script but DISPLAY var is empty
++++ sudo service lightdm restart
++++ export_display_variable
+++++ ls /tmp/.X11-unix/
+++++ sed -e 's@^X@:@'
++++ [[ '' == '' ]]
++++ echo 'Imposible to get DISPLAY variable. Check your system'
Imposible to get DISPLAY variable. Check your system
++++ exit 1
Build step 'Execute shell' marked build as failure

When debugging jobs in the new cloud linux agents. We faced an issue with the DISPLAY environment variable. It seems it's caused when we restart lightdm service using sudo service lightdm restart and then running export_display_variable.

In a conversation with @nuclearsandwich he mentioned that this issue could be because of a race condition in the lightdm restart scripts mainly because it needs to wait an extra few seconds for X11 to come up. This could make the first build of an agent fail sometimes.

Reference lines:

• sudo sercive lightdm restart
• export_display_variable