bug#25476: pivot-root test fails on Ubuntu 16.04

[Paul Garlick]

> I think a1a8b7f2e20513a3ad968e74e7ec52546404e3c6 fixes this.
> 
> 
Yes, confirmed.  The utmpx-entries test now passes.
The pivot-root test still fails.  One thought; I made a change to the
guix-daemon.service file, at about the same time as updating Ubuntu to
16.04, setting TMPDIR to /data/tmp.  Could that be related?
Paul.