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
  4a180da
  c46d4da

nixos/modules/services/x11/display-managers/gdm.nix

@@ -159,6 +159,10 @@ in
       "rc-local.service"
       "systemd-machined.service"
       "systemd-user-sessions.service"
+      "getty@tty1.service"
+    ];
+    systemd.services.display-manager.conflicts = [
+      "getty@tty1.service"
     ];
 
     systemd.services.display-manager.serviceConfig = {