You can’t sign in or sign out of an app while you are in the builder. You can preview the app as different users, but since you are designing and building the app, there is no need to sign in. Technically, while you are in the builder, you are always signed into the app.
Is there a reason why you need it to go to the sign in screen while you are in the builder? It will work correctly with the published app. If you want to see, or make changes to the sign in screen, you can access that through the app settings.