nixos/gnome-initial-setup: prevent run on existing systems

GNOME initial setup's run is conditioned on whether
the gnome-initial-setup-done file exists in XDG_CONFIG_HOME
Because of this, every existing user will have initial setup
running because they never ran it before.

To prevent this we create the file if the users stateVersion
is older than 20.03 (the release we added this module).

Also drop uneeded manual conflicts as systemd.packages
does handle this.