Make workspace relocatable by using relative subdirectory. (#34)

Fix #33

According to colcon-core, the subdirectory parametre should be a relative
path to use the COLCON_CURRENT_PREFIX and make the workspace relocatable.

Author: romainreignier

colcon_ros_cargo/task/ament_cargo/build.py

@@ -83,7 +83,7 @@ def _prepare(self, env, additional_hooks):
             Path(self.context.args.install_base),
             self.context.pkg.name,
             'AMENT_PREFIX_PATH',
-            self.context.args.install_base,
+            '',
             mode='prepend')
 
     def _build_cmd(self, cargo_args):