jenkins.sh: fix the download url if the qemu image wasn't setup

Download the image from yesterday because linuxcontainers only contains the images of the last 3 days.

Change-Id: I75e270b7255c1cd7fca1674111b0f19fc7bba74f
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index cee709d..8751abd 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -41,7 +41,8 @@
                                 sed -i "s/setup_proc//g" "$ROOTFS/debootstrap/suite-script"
                                 mychroot /debootstrap/debootstrap --second-stage --verbose http://ftp.de.debian.org/debian/
                         else
-                                wget -nc -q "https://uk.images.linuxcontainers.org/images/debian/stretch/armhf/default/20180114_22:42/rootfs.tar.xz"
+                                YESTERDAY=$(python -c 'import datetime ; print((datetime.datetime.now() - datetime.timedelta(days=1)).strftime("%Y%m%d"))')
+                                wget -nc -q "https://uk.images.linuxcontainers.org/images/debian/stretch/armhf/default/${YESTERDAY}_22:42/rootfs.tar.xz"
                                 tar -xf rootfs.tar.xz -C "$ROOTFS/" || true
                                 echo "nameserver 8.8.8.8" > "$ROOTFS/etc/resolv.conf"
                         fi