C#: set some dotnet options when testing

kokoro/release/csharp/windows/build_nuget.bat

@@ -7,4 +7,8 @@ cd csharp
 powershell -File install_dotnet_sdk.ps1
 set PATH=%LOCALAPPDATA%\Microsoft\dotnet;%PATH%
 
+@rem Disable some unwanted dotnet options
+set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
+set DOTNET_CLI_TELEMETRY_OPTOUT=true
+
 call build_packages.bat

kokoro/windows/csharp/build.bat

@@ -7,4 +7,8 @@ cd csharp
 powershell -File install_dotnet_sdk.ps1
 set PATH=%LOCALAPPDATA%\Microsoft\dotnet;%PATH%
 
+@rem Disable some unwanted dotnet options
+set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
+set DOTNET_CLI_TELEMETRY_OPTOUT=true
+
 call buildall.bat

tests.sh

@@ -117,6 +117,12 @@ build_csharp() {
   internal_build_cpp
   NUGET=/usr/local/bin/nuget.exe
 
+  # Disable some unwanted dotnet options
+  export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
+  export DOTNET_CLI_TELEMETRY_OPTOUT=true
+
+  # TODO(jtattermusch): is this still needed with "first time experience"
+  # disabled?
   # Perform "dotnet new" once to get the setup preprocessing out of the
   # way. That spews a lot of output (including backspaces) into logs
   # otherwise, and can cause problems. It doesn't matter if this step