util: allow transmitting XDG_* variables directly

For Braille output to be properly dispatched, Orca needs the XDG_VTNR variable. 646b9bc0 ('util: Blacklist some session-specific variables') broke this by blacklisting the variable along others. The git log talks about leaking from one session to another, but this seems to have been fixed, and can not be reproduced with e.g Debian bullseye as of today.