The path $TMP_BUILD_DIR/mnt becomes the / inside the chroot during
the chroot phases of diskimage-builder. Previously this path was being
created using the account running diskimage-builder. This account may
not be valid inside the chroot. This causes path validation, when running
on a Ubuntu bionic host, to fail.
This patch chown's the $TMP_BUILD_DIR/mnt to root.root to make sure
that / is owned by a valid account inside the chroot.
Change-Id: Ifedc136baa67c7952942aed2c8cb1041902fef91
Closes-Bug: 1811113