Fix nonexistent script test in windows

sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart

Index: sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart b/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
index 973844456242f12eff40ca852cb698180b1e81b9..7704fdf6ba4cd7cc37d5354796582940b7c4d15c 100644
--- a/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
+++ b/sdk/lib/_internal/pub/test/run/nonexistent_script_test.dart
@@ -15,7 +15,7 @@ main() {
       d.appPubspec()
     ]).create();
 
-    var pub = pubRun(args: ["bin/script"]);
+    var pub = pubRun(args: [p.join("bin", "script")]);
     pub.stderr.expect("Could not find ${p.join("bin", "script.dart")}.");
     pub.shouldExit(exit_codes.NO_INPUT);
   });