enable competition bridges when world uses absolute paths

[M1chaelM]

A simple fix to address problems with task-specific bridges failing, described in #680 and osrf/vrx-docker#60

To test

First observe that task specific bridges are currently failing if the competition world is specified using an absolute path:

cd vrx_ws
cp src/vrx/vrx_gz/worlds/2023_practice/practice_2023_stationkeeping0_task.sdf stationkeeping0.sdf
ros2 launch vrx_gz competition.launch.py world:=`pwd`/stationkeeping0

Note that gz topic -l shows

/vrx/stationkeeping/goal
/vrx/stationkeeping/mean_pose_error
/vrx/stationkeeping/pose_error

but ros2 topic list does not.

Now checkout this branch, build again, and re-run the launch command above. Verify that the competition topics now show up.

[caguero]

Works for me as expected.