Can't load plugin with roslaunch

edit

My world file (adapted from the tutorial) contains a plugin :

<model name="my_car">
    <include>
        <uri>model://car</uri>
    </include>
    <plugin name="car_control" filename="car_plugin/build/libcar_plugin.so">
        <velocity>2</velocity>
    </plugin>
</model>

What works

The plugin loads and displays the expected message when I do this :

$ export GAZEBO_MODEL_PATH="$CATKIN_WS/src/tas_group_2/car_gazebo/models"
$ export GAZEBO_PLUGIN_PATH="$CATKIN_WS/src/tas_group_2/car_gazebo/plugins"
$ gazebo worlds/car.world

What doesn't

However I can't get it to launch through roslaunch with :

$ roslaunch car_gazebo car.launch

and I get te following error :

[Err] [Plugin.hh:165] Failed to load plugin car_plugin/build/libcar_plugin.so: car_plugin/build/libcar_plugin.so: cannot open shared object file: No such file or directory

My files contain : package.xml (car_gazebo) :

<?xml version="1.0"?>
<package>
    <name>car_gazebo</name>
        ...
    <run_depend>gazebo_ros</run_depend>
    <build_depend>gazebo_ros</build_depend>
    <buildtool_depend>catkin</buildtool_depend>

    <export>
        <gazebo_ros gazebo_model_path="${prefix}/models"/>
        <gazebo_ros gazebo_plugin_path="${prefix}/plugins"/>
    </export>
</package>

car.launch :

<launch>
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find car_gazebo)/worlds/car.world" />
    <arg name="verbose" value="true" />
  </include>
</launch>

My question

I thought the two methods where equivalent. Because I specified the environment variables in the <export> tag.

<gazebo_ros gazebo_model_path="${prefix}/models"/> allowed gazebo to find my model, while <gazebo_ros gazebo_plugin_path="${prefix}/plugins"/> doesn't help find my plugin. How come?

Any idea how to get the plugin to work with the roslaunch command?

Cheers