base-test: Correct filter on Zuul var YAML export

Other examples I find use | instead of || so let's give that a shot.
Current use is erroring with:

    template error while templating string: expected token 'name',
    got '|'. String: {{ zuul || to_nice_yaml }}

Change-Id: Ie8a9786ceb3888f78ce1b51eba621a0f04158fee

playbooks/base-test/pre.yaml

@@ -32,7 +32,7 @@
     copy:
       dest: "{{ zuul.executor.log_root }}/zuul-info/zuul_vars.yaml"
       mode: 0644
-      content: "{{ zuul || to_nice_yaml }}"
+      content: "{{ zuul | to_nice_yaml }}"
 # <--
 
 - hosts: all