fix jenkins build on windows

tools/jenkins/run_jenkins.sh

@@ -50,6 +50,9 @@ elif [ "$platform" == "windows" ]
 then
   echo "building $language on Windows"
 
+  # Prevent msbuild from picking up "platform" env variable, which would break the build
+  unset platform
+
   # TODO(jtattermusch): integrate nuget restore in a nicer way.
   /cygdrive/c/nuget/nuget.exe restore vsprojects/grpc.sln
   /cygdrive/c/nuget/nuget.exe restore src/csharp/Grpc.sln