Windows tweak

tools/run_tests/run_tests.py

@@ -281,7 +281,10 @@ class CSharpLanguage(object):
   def make_targets(self):
     # For Windows, this target doesn't really build anything,
     # everything is build by buildall script later.
-    return ['grpc_csharp_ext']
+    if self.platform == 'windows':
+      return []
+    else:
+      return ['grpc_csharp_ext']
 
   def build_steps(self):
     if self.platform == 'windows':