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

pkgs/desktops/gnome-3/core/gdm/default.nix

···
   configureFlags = [ "--sysconfdir=/etc"
                      "--localstatedir=/var"
                      "--with-plymouth=yes"
+                     "--with-initial-vt=7"
                      "--with-systemdsystemunitdir=$(out)/etc/systemd/system" ];
 
   nativeBuildInputs = [ autoreconfHook pkgconfig ];