system: Fix which()

abspath was used instead of isabs, this broke the logic.

Change-Id: Ida05d44b057056ffbd6300ab4393e143a0ba79b3
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=991408
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>