gnome3.gnome-session: always run /etc/set-environment on startup

gnomes-session sources its environment from a login shell. But if the systemd
user environment already contains `__NIXOS_SET_ENVIRONMENT_DONE` the environment
setup won't happen correctly. Simply unset this variable to ensure a fresh
environment.

This was previously handled by a GDM patch, but it no longer works reliably with
gnome-sessions systemd session.

We still need the GDM patch to handle running other sessions (eg. plasma).

See https://github.com/NixOS/nixpkgs/issues/48255 for more info on the
underlying issue.