Fix group routes custom root not-found (#54931)
For group routes, as their custom layout show up at the 1st level of the loader tree, so previously we tried to add the default not-found to that layer, but if they have custom root not-found, we should use that as the root not-found componeny, instead of using default not-found component to form the 404 page
Closes NEXT-1588
Fixes #51075
Fixes #54892
