Use --unpretty=expanded instead of --pretty

Author: jyn514

ui/src/sandbox.rs

@@ -399,7 +399,7 @@ impl Sandbox {
             "rustc",
             "--",
             "-Zunstable-options",
-            "--pretty=expanded",
+            "--unpretty=expanded",
         ]);
 
         log::debug!("Macro expansion command is {:?}", cmd);