Diskimage-builder fails to build ubuntu-minimal images when run on
a Ubuntu bionic-beaver (18.04) instance.
The user gets "Couldn't create tempfiles for splitting up" when
apt-get update is run in the ubuntu-minimal element root.d.
The issue is that the /tmp inside the chroot is not getting the
proper permissions applied from the base-files package. This is likely
because the pip-cache element has already created the directory before
the base-files package is installed.
This patch changes the order of pip-cache to root.d/11-pip-cache so that
it runs after teh base OS root.d elements run.
Change-Id: I6fd1cb2a23422206884165eb502b260f0c1e52f7