Fix incorrect variable usage in when statement

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/868

roles/matrix-postgres-backup/tasks/main.yml

@@ -5,7 +5,7 @@
     - always
 
 - import_tasks: "{{ role_path }}/tasks/validate_config.yml"
-  when: "run_setup|bool and matrix_postgres_enabled|bool"
+  when: "run_setup|bool and matrix_postgres_backup_enabled|bool"
   tags:
     - setup-all
     - setup-postgres-backup