Set env for systemctl --user

[Chris Warburton]

The pulseaudio service runs as the current user (chris), so if it fails
(e.g. missing bluez, jackd, whatever) it needs to be restarted with:

    systemctl --user restart pulseaudio

This doesn't work, since it says "can't connect to bus".

The solution given here works https://askubuntu.com/a/813644/470416

Basically, we need to set the XDG_RUNTIME_DIR env var:

    XDG_RUNTIME_DIR=/run/user/1000 systemctl --user restart pulseaudio

The above command works.

So, we should ensure that this env var is set in the user's
environment/shell/profile/whatever.