nixos/gdm: start after getty@tty1

GDM is now killed if tty1 is started after gdm is launched. This follows
upstream's gdm service config.

This might cause problems with nixos-rebuild switch though. See the reasoning
and work that led to not following upstream on this:
  https://github.com/NixOS/nixpkgs/issues/21439
  4a180dac
  c46d4dab