build: Also pull in ostree from updates-testing

To test out composefs faster.
xref coreos/fedora-coreos-config#2856 (comment)

Author: cgwalters

build.sh

@@ -67,7 +67,7 @@ install_rpms() {
 
     # Add fast-tracked packages here.  We don't want to wait on bodhi for rpm-ostree
     # as we want to enable fast iteration there.
-    yum -y --enablerepo=updates-testing upgrade rpm-ostree
+    yum -y --enablerepo=updates-testing upgrade rpm-ostree ostree
 
     # Delete file that only exists on ppc64le because it is causing
     # sudo to not work.