Provide ZUUL_BRANCH in constraints generation job

The constraints generation job script uses zuul-cloner, so set
ZUUL_BRANCH since it's run from a periodic pipeline where Zuul
doesn't provide the ZUUL_BRANCH nor ZUUL_REF job parameters
zuul-cloner requires.

Change-Id: I65d3aa38eb45cfbecf13561d3af4b4e0235f2f5d

jenkins/jobs/requirements.yaml

@@ -96,8 +96,9 @@
       - branch-git-prep:
           branch: '{branch-override}'
       - shell: |
+          export ZUUL_BRANCH={branch-override}
           /usr/local/jenkins/slave_scripts/propose_update.sh \
-          requirements-constraints
+              requirements-constraints
 
     publishers:
       - console-log