update prepare scripts

tools/internal_ci/helper_scripts/prepare_build_interop_rc

@@ -28,6 +28,7 @@ git clone --recursive https://github.com/grpc/grpc-go ./../grpc-go
 git clone --recursive https://github.com/grpc/grpc-java ./../grpc-java
 git clone --recursive https://github.com/grpc/grpc-node ./../grpc-node
 git clone --recursive https://github.com/grpc/grpc-dart ./../grpc-dart
+git clone --recursive https://github.com/grpc/grpc-dotnet ./../grpc-dotnet
 
 # Download json file.
 mkdir ~/service_account

tools/internal_ci/helper_scripts/prepare_build_macos_interop_rc

@@ -22,3 +22,4 @@ git clone --recursive https://github.com/grpc/grpc-go ./../grpc-go
 git clone --recursive https://github.com/grpc/grpc-java ./../grpc-java
 git clone --recursive https://github.com/grpc/grpc-node ./../grpc-node
 git clone --recursive https://github.com/grpc/grpc-dart ./../grpc-dart
+git clone --recursive https://github.com/grpc/grpc-dotnet ./../grpc-dotnet