[gnome-builder] cargo: fix creation of wrapper build target

[Christian Hergert <chergert src gnome org>]

commit 67c04157fd8811de9adf64ad23151597974dddf8
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jan 21 10:27:06 2019 -0800

    cargo: fix creation of wrapper build target

 src/plugins/cargo/cargo_plugin.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/plugins/cargo/cargo_plugin.py b/src/plugins/cargo/cargo_plugin.py
index 690725354..7fc7b14b4 100644
--- a/src/plugins/cargo/cargo_plugin.py
+++ b/src/plugins/cargo/cargo_plugin.py
@@ -198,7 +198,7 @@ class CargoBuildTargetProvider(Ide.Object, Ide.BuildTargetProvider):
                                          code=Gio.IOErrorEnum.NOT_SUPPORTED))
             return
 
-        task.targets = [CargoBuildTarget(context=self.get_context())]
+        task.targets = [build_system.ensure_child_typed(CargoBuildTarget)]
         task.return_boolean(True)
 
     def do_get_targets_finish(self, result):