Work around pbuilder not properly setting $USER

On su'ing to the build user, pbuilder only sets $LOGNAME to the build
user's name ("pbuilder"), but leaves $USER as is, i. e. "root"
(cf. http://bugs.debian.org/677666).  This confuses the test suite.

This change imitates the behaviour of jsub itself: Try $LOGNAME before
using $USER; thus the test suite succeeds when run under pbuilder.

Change-Id: Ia0553ecf802082609e61c5ddb0e6546719297417