run-production-playbook-post: generate timestamp

When this moved with I3e99b80e442db0cc87f8e8c9728b7697a5e4d1d3 we lost
access to the variable set as a fact; regenerate it.  In a future
change we can look at strategies to share this with the start
timestamp (not totally simple as it is across playbooks on a
dynamicaly added host).

Change-Id: Ic18c89ecaf144a69e82cbe9eeed2641894af71fb

playbooks/zuul/run-production-playbook-post.yaml

@@ -83,6 +83,10 @@
 
     # If we aren't publishing logs through zuul then keep a set on
     # bridge directly.
+    - name: Get a current timestamp
+      set_fact:
+        _log_timestamp: "{{ lookup('pipe', 'date +%Y-%m-%dT%H:%M:%S') }}"
+
     - name: Rename playbook log on bridge
       when: not infra_prod_playbook_collect_log
       become: yes