Fix C# nuget build on Kokoro (#4912)

* upgrade dotnet SDK to 2.1.3 on kokoro linux

csharp/global.json

 {
   "sdk": {
-    "version": "2.0.3"
+    "version": "2.1.3"
   }
 }

kokoro/linux/prepare_build_linux_rc

@@ -7,7 +7,7 @@ sudo sh -c 'echo "deb [arch=amd64] https://packages.microsoft.com/repos/microsof
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EB3E94ADBE1229CF
 sudo apt-get update
 # We use the .NET Core SDK 2.x to build...
-sudo apt-get install -y dotnet-sdk-2.0.3
+sudo apt-get install -y dotnet-sdk-2.1.3
 # But we also need the 1.x framework to test against, as we
 # target netstandard1.x
 sudo apt-get install -y dotnet-sharedframework-microsoft.netcore.app-1.0.5

kokoro/release/csharp/windows/build_nuget.bat

 @rem enter repo root
 cd /d %~dp0\..\..\..\..
 
-cd csharp\src
+cd csharp
 call build_packages.bat