gdm: start gdm on tty7 rather than default tty1
gdm: start gdm on tty7 rather than default tty1

- Fixes nixos-rebuild switch/test issue with GNOME
  https://github.com/NixOS/nixpkgs/issues/21439
- The solution was given here:
  https://github.com/deedrah/nixpkgs/commit/d761e66a41e09bb0a1ebe31612f3678f9ba801a1#commitcomment-25382880

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