Fix yumdownloader cache dir

TMPDIR env is not being honored as cache dir in EL 8.

Change-Id: I8281675ec5f0951b3e190a8d6727744a1a5cd8d7

diskimage_builder/elements/yum-minimal/root.d/08-yum-chroot

@@ -119,6 +119,7 @@ function _install_repos {
     TMPDIR=${temp_tmp} yumdownloader --verbose \
         --releasever=$DIB_RELEASE \
         --setopt=reposdir=$repo \
+        --setopt=cachedir=$temp_tmp \
         --destdir=$WORKING \
         ${packages} && rc=$? || rc=$?
     rm -rf ${temp_tmp}