Fix branch override in zuul-branch-git-prep

Rather than overriding ZUUL_BRANCH (which should not be overidden)
set the --branch argument to zuul cloner, which is the z-c equivalent
of OVERRIDE_BRANCH.

Change-Id: Ie40b668a3a33943a0431047d18716329c1e440c0

jenkins/jobs/macros.yaml

@@ -81,10 +81,10 @@
           export ZUUL_REF=${{ZUUL_REF:-None}}
           export BRANCH_OVERRIDE={branch-override}
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
-              export ZUUL_BRANCH=$BRANCH_OVERRIDE
+              BRANCH_ARG="--branch $BRANCH_OVERRIDE"
           fi
           /usr/zuul-env/bin/zuul-cloner -m $CLONEMAP --cache-dir /opt/git \
-              git://git.openstack.org $ZUUL_PROJECT
+              $BRANCH_ARG git://git.openstack.org $ZUUL_PROJECT
 
 - builder:
     name: branch-git-prep