gnome3.gdm: fix session chooser

We are patching GDM to respect GDM_SESSIONS_DIR environment
variable, which we are setting in the GDM module. Previously, we
only took care of a single code path, the one that handled session
start-up; missing the one obtaining the list of sessions.

This commit patches the second code path, and also whitelists the
GDM_SESSIONS_DIR so that it can be passed to the greeter.

Fixes #34101