98-clean-chroot: check existence of aptitude binary via $ROOTCMD.

/usr/bin/aptitude might point to /etc/alternatives/aptitude which
points to e.g. /usr/bin/aptitude-curses. When testing for
precense of /usr/bin/aptitude the test might fail because test(1)
dereferences symbolic links. So make sure to check existence of
aptitude binary inside $target using $ROOTCMD.