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.
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