dbus-update-activation-environment and systemctl --user import-environment
were fired asynchronously via spawn_async_no_shell, racing with the autostart
script. Any systemd user service started from autostart (e.g. via
labwc-session.target) could start before the import completed, leaving
WAYLAND_DISPLAY and related variables absent from its environment and those
of any apps it launches.
Run both commands synchronously via a new spawn_sync_no_shell helper so
the import is guaranteed to complete before the autostart script executes.
998fd80737