Fix order to be populated in adjustments on admin actions

@dkubb `Order#adjustments` is one side of a polymorphic association for that reason `order_id` does not get populated automatically. `#build` and `#new` are aliases of each other on polymorphic associations.

Another PR I'm preparing will promote `Order#all_adjustments` to be a real non polymorphic association, there we can use `#build` to simplify things.