Fixes <http://bugs.gnu.org/17946>.
Notoriously, cron jobs may set LOGNAME only and not USER. See
e.g. crontab(5) under Debian 7 (wheezy).
* guix/scripts/package.scm (%profile-directory)
(guix-package): Also try LOGNAME if USER is unset.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>