env: Set systemd unit and generator paths

This will enable gnome-session!34 (merged) to work correctly inside jhbuild environments.

To make all this work, we need a proper wrapper script. I have a working script (that even works with older systemd versions). My plan is to ship that with gnome-session as it could be useful for a number of purposes. Like setting up multiple parallel test environments.

I am planning on updating this MR with the relevant jhbuild-session script changes to make use of everything.