[gnome-builder] cargo: fix format for cargo arguments

[Christian Hergert <chergert src gnome org>]

commit 7e2fa0a92ca01f638ead822951352f1e1cd269a8
Author: Christian Hergert <chergert redhat com>
Date:   Sun Feb 17 16:06:29 2019 -0800

    cargo: fix format for cargo arguments

 src/plugins/cargo/cargo_plugin.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/src/plugins/cargo/cargo_plugin.py b/src/plugins/cargo/cargo_plugin.py
index ea2b6e5e9..3855f67a1 100644
--- a/src/plugins/cargo/cargo_plugin.py
+++ b/src/plugins/cargo/cargo_plugin.py
@@ -133,8 +133,7 @@ class CargoPipelineAddin(Ide.Object, Ide.PipelineAddin):
         build_launcher.push_argv('--')
         build_launcher.push_argv('--error-format')
         build_launcher.push_argv('short')
-        build_launcher.push_argv('--remap-path-prefix')
-        build_launcher.push_argv('=' + pipeline.get_srcdir())
+        build_launcher.push_argv('--remap-path-prefix=%s' % pipeline.get_srcdir())
 
         clean_launcher = pipeline.create_launcher()
         clean_launcher.setenv('CARGO_TARGET_DIR', builddir, True)