buildinstall: No copy if task fails
Even a failed lorax task can leave behind some in-progress images. Pungi
needs to copy the files into compose/ subdirectory only if the task
finished successfully.
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1485021
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>