make build_packages.bat provide version of grpc.native.csharp_ext dependency

src/csharp/Grpc.Core/Grpc.Core.nuspec

@@ -17,7 +17,7 @@
 	<dependencies>
 	  <dependency id="System.Collections.Immutable" version="1.1.36" />
 	  <dependency id="Ix-Async" version="1.2.3" />
-	  <dependency id="grpc.native.csharp_ext" version="0.10.0" />
+	  <dependency id="grpc.native.csharp_ext" version="$GrpcNativeCsharpExtVersion$" />
     </dependencies>
   </metadata>
   <files>

src/csharp/build_packages.bat

@@ -16,7 +16,7 @@ endlocal
 
 %NUGET% pack ..\..\vsprojects\nuget_package\grpc.native.csharp_ext.nuspec -Version %CORE_VERSION% || goto :error
 %NUGET% pack Grpc.Auth\Grpc.Auth.nuspec -Symbols -Version %VERSION% || goto :error
-%NUGET% pack Grpc.Core\Grpc.Core.nuspec -Symbols -Version %VERSION% || goto :error
+%NUGET% pack Grpc.Core\Grpc.Core.nuspec -Symbols -Version %VERSION% -Properties GrpcNativeCsharpExtVersion=%CORE_VERSION% || goto :error
 %NUGET% pack Grpc.HealthCheck\Grpc.HealthCheck.nuspec -Symbols -Version %VERSION% || goto :error
 %NUGET% pack Grpc.Tools.nuspec -Version %VERSION% || goto :error
 %NUGET% pack Grpc.nuspec -Version %VERSION% || goto :error