Setting user environment variables on Windows is very slow

[agocke]

See https://superuser.com/questions/565771/setting-user-environment-variables-is-very-slow for another example.

The problem is this line: https://github.com/dotnet/runtime/blob/a87fd8bc2fe1bfa3a96d85b05d20a8341c1eb895/src/libraries/System.Private.CoreLib/src/System/Environment.Win32.cs#L58

Sometimes notifying other applications is very slow.

This could be solved specifically in dnvm by extracting just the registry-set capability and ignoring the notification.