Pass environment variables to 'tox envlist config' task

As of version 3.21.0, tox now throws an exception if an environment variable
used in config is missing. We now need to pass the variables also to the task
"Get tox envlist config".

Change-Id: I4073f55b6ba3456f3d2db40cafb692efbb478914

roles/tox/tasks/siblings.yaml

@@ -25,6 +25,7 @@
     > {{ _tox_show_config_tempfile.path }}
   args:
     chdir: "{{ zuul_work_dir }}"
+  environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
 
 - name: Install any sibling python packages
   tox_install_sibling_packages: