Add root_marker_dir for releasenotes publish

To use the write-root-marker role from opendev/base-jobs, set
the root_marker_dir to the same value that is currently set
by the openstack-zuul-jobs role.

Needed-By: https://review.opendev.org/731460
Change-Id: I169f069aa458bc7d02f9c0295702c1316fff825f

playbooks/publish/releasenotes.yaml

@@ -1,6 +1,7 @@
 - hosts: all
   roles:
     - role: write-root-marker
+      root_marker_dir: "{{ zuul_work_dir }}/{{ sphinx_build_dir }}/html"
       when: zuul_success | bool
     - role: fetch-sphinx-output
       sphinx_output_suffix: "/"