pick up nuget package version from cmdline param and fix nuget build

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Auth/Grpc.Auth.nuspec

src/csharp/Grpc.Core/Grpc.Core.csproj

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

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.csproj

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.nuspec

src/csharp/Grpc.Tools.nuspec

src/csharp/Grpc.nuspec

src/csharp/build_packages.bat