Cleanup yumdownloader repos

yumdownloader has to have all the repo XML files, etc, which adds up
to a not totally insignificant 150MiB or so.  Currently we're leaking
this directory for every build, which adds up on regualar builders
like nodepool.

Isolate the call with a separate TMPDIR so we can clean it up after
the initial download.

Change-Id: Ic65e8ca837cc76b7a1bb9f83027b4a5bdd270f75

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

@@ -56,6 +56,7 @@ _RPM="rpm --dbpath=/var/lib/rpm"
 # has yum/yumdownloader
 function _install_repos {
     local packages
+    local rc
 
     # pre-install the base system packages via rpm.  We previously
     # just left it up to yum to drag these in when we "yum install
@@ -79,11 +80,21 @@ function _install_repos {
         packages+="${DISTRO_NAME}-repos "
     fi
 
-    yumdownloader \
+    # yumdownloader puts repo xml files and such into a directory
+    # ${TMPDIR}/yum-$USER-random.  Since we don't need this once the
+    # initial download happens, redirect TMPDIR for this call so we
+    # can clean it up nicely
+    local temp_tmp
+    temp_tmp=$(mktemp -d)
+    TMPDIR=${temp_tmp} yumdownloader \
         --releasever=$DIB_RELEASE \
         --setopt=reposdir=$TMP_HOOKS_PATH/yum.repos.d \
         --destdir=$WORKING \
-        ${packages}
+        ${packages} && rc=$? || rc=$?
+    rm -rf ${temp_tmp}
+    if [[ ${rc} != 0 ]]; then
+        die "Failed to download initial packages: ${packages}"
+    fi
 
     # --nodeps works around these wanting /bin/sh in some fedora
     # releases, see rhbz#1265873