Create YUM_CACHE_DIR in yum-minimal

yum-minimal/root.d/08-yum-chroot runs before yum/root.d/50-yum-cache,
and thus if run on a completely fresh system will fail in
08-yum-chroot as the YUM_CACHE directory isn't made.

This is probably hidden by testing & nodepool builds, because it sets
DIB_IMAGE_CACHE.  It was hidden from me because locally I have done
builds using the "yum" element previously, which had created the
cache.

Change-Id: I333f5f7e67d198f75a522cc296c118c2e94a5ecb

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

@@ -38,6 +38,7 @@ EACTION="rm -r $WORKING"
 trap "$EACTION" EXIT
 
 YUM_CACHE=$DIB_IMAGE_CACHE/yum
+mkdir -p $YUM_CACHE
 
 # install the [fedora|centos]-[release|repo] packages inside the
 # chroot, which are needed to bootstrap yum/dnf