Add sleep to workaround e2fsck errors

LP: 1960537 illustrates an issue where the calls to e2fsck in the
umount_partition call are failing due to an open file handle. At this
time, we are unable to find a root cause, and it's causing many builds
to fail for CPC. Adding a sleep 30 as a workaround as the file handle
releases within  that timeframe. This does not address root cause.

live-build/functions

@@ -234,6 +234,8 @@ umount_partition() {
     mount --make-private $mountpoint
     umount $mountpoint
     udevadm settle
+    # workaround for LP: 1960537
+    sleep 30 
 
     if [ -n "${rootfs_dev_mapper}" -a -b "${rootfs_dev_mapper}" ]; then
         # buildd's don't have /etc/mtab symlinked