[gnome-builder] cargo: Cargo.toml needs to be a path string

[Christian Hergert <chergert src gnome org>]

commit 4aaeed60f5387cece076526022ac30716c36198b
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jan 21 10:32:48 2019 -0800

    cargo: Cargo.toml needs to be a path string
    
    Now that the build target is rooted, we can fix the real issue, which is
    a file being passed when a string is expected.
    
    Fixes #772

 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 815e017f8..248b8e42b 100644
--- a/src/plugins/cargo/cargo_plugin.py
+++ b/src/plugins/cargo/cargo_plugin.py
@@ -179,7 +179,7 @@ class CargoBuildTarget(Ide.Object, Ide.BuildTarget):
 
         # Pass the Cargo.toml path so that we don't
         # need to run from the project directory.
-        cargo_toml = context.ref_workdir().get_child('Cargo.toml')
+        cargo_toml = context.ref_workdir().get_child('Cargo.toml').get_path()
 
         return [cargo, 'run', '--manifest-path', cargo_toml]