Without this, even if you configure a preference for Electron apps to
use Ozone by setting `NIXOS_OZONE_WL=1`, GUI apps launched through
systemd user services use XWayland, since the variable is not set in
their environment.
This fixes that issue by importing it, like we do other variables.
- The `XDG_SESSION_TYPE` environment variable is used by some applications and frameworks to
detect wayland sessions (i.e qt5/6, electron/chromium). It is set by wlroots since version 0.13.0 [1].
- Propagating `XDG_SESSION_TYPE` to the systemd user environment is necessary when processes launched by
services (e.g emacs) need to inherit the environment variable.
[1] - 90c8452959
