move C# nuget restore on linux to prebuild phase
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index 6423ecf..69a480a 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -321,7 +321,10 @@
             for assembly in assemblies]
 
   def pre_build_steps(self):
-    return []
+    if self.platform == 'windows':
+      return []
+    else:
+      return [['tools/run_tests/pre_build_csharp.sh']]
 
   def make_targets(self):
     # For Windows, this target doesn't really build anything,