Adding -x to set parameters for more output

When the cache tarball gets fail the output doesn't indicate
what url is failing.  Adding '-x' displays the urls attempted.

Change-Id: I71b2dbfd712e72c591410ca9b68edc76d493ad6e

elements/ubuntu/root.d/10-cache-ubuntu-tarball

@@ -1,7 +1,7 @@
 #!/bin/bash
 # These are useful, or at worst not harmful, for all images we build.
 
-set -eu
+set -eux
 set -o pipefail
 
 [ -n "$ARCH" ]