Fix C-core running under 5.0 in benchmark environment (#569)

Author: JamesNK

perf/benchmarkapps/GrpcCoreServer/GrpcCoreServer.csproj

@@ -1,8 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework Condition="'$(BenchmarksTargetFramework)' != ''">$(BenchmarksTargetFramework)</TargetFramework>
+    <OutputType>Exe</OutputType>
   </PropertyGroup>
 
   <ItemGroup>