Make the C# benchmarks non-packable

csharp/src/Google.Protobuf.Benchmarks/Google.Protobuf.Benchmarks.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp2.1</TargetFramework>
+    <IsPackable>False</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>