xss-lock needs XDG_SESSION_ID to respond to loginctl lock-session(s)
(and possibly other session operations such as idle hint management).
This change adds XDG_SESSION_ID to the list of imported environment
variables when starting systemctl.
Inspired by home-manager, add importVariables configuration.
Set session to XDG_SESSION_ID when running xss-lock as a service.
Co-authored-by: misuzu <bakalolka@gmail.com>