Revert "pam_systemd: drop setting DBUS_SESSION_BUS_ADDRESS"

This reverts commit 2b2b7228.

Fixes #11293.

Removing the environment variable causes problems, e.g. Xfce and Chromium and
... don't communicate with the running dbus instance. If they attempt to start their
own instance, things become even more confusing. Those packages could be fixed
one by one, but removing the variable right now is causing too many problems.