build-pdf-docs: Pass environment

Run pdf-docs with same environment that is used for the --notest run.

Change-Id: I5b6a263e01e574097f03011eaca768730701da97

roles/build-pdf-docs/tasks/main.yaml

@@ -17,6 +17,7 @@
 - name: Run tox with output redirected to a file
   args:
     chdir: "{{ zuul_work_dir }}"
+  environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
   shell: |
     echo "Running tox -e {{ tox_pdf_envlist }} -vv > {{ ansible_user_dir }}/zuul-output/logs/sphinx-build-pdf.log 2>&1"
     echo "Output is redirected to a file as it tends to be quite verbose."