fix problem when restoring C# dependencies on win2016 (#8243)

kokoro/release/csharp/windows/build_nuget.bat

@@ -11,4 +11,7 @@ set PATH=%LOCALAPPDATA%\Microsoft\dotnet;%PATH%
 set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
 set DOTNET_CLI_TELEMETRY_OPTOUT=true
 
+@rem Work around https://github.com/dotnet/core/issues/5881
+dotnet nuget locals all --clear
+
 call build_packages.bat

kokoro/windows/csharp/build.bat

@@ -11,4 +11,7 @@ set PATH=%LOCALAPPDATA%\Microsoft\dotnet;%PATH%
 set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
 set DOTNET_CLI_TELEMETRY_OPTOUT=true
 
+@rem Work around https://github.com/dotnet/core/issues/5881
+dotnet nuget locals all --clear
+
 call buildall.bat