gnome3.gdm: Always reset the environment for new sessions

When GDM launches a new session it will inherit the user's systemd
environment (but only unset variables). If `__NIXOS_SET_ENVIRONMENT_DONE` is set
in the user's systemd environment it will prevent the environment to be set
properly or updated (eg. after having done a system rebuild).

Gnome sessions exports their environment to systemd at startup. If something
is keeping the user's systemd process alive (eg. ssh) launching a new gnome
session after logging out will result in a broken PATH. Specifically the PATH
will be inherited from GDM and never reset.

We patch GDM to never inherit `__NIXOS_SET_ENVIRONMENT_DONE` so new sessions
will always reset their base environment.

fixes https://github.com/NixOS/nixpkgs/issues/48255

For more info about the environment setup:
https://github.com/NixOS/nixpkgs/pull/45784