fix wrong variable name

roles/matrix-riot-web/tasks/setup_riot_web.yml

@@ -40,7 +40,7 @@
       dockerfile: Dockerfile
       path: "{{ matrix_riot_web_docker_src_files_path }}"
       pull: yes
-  when: "matrix_riot_web_enabled|bool and matrix_self_build"
+  when: "matrix_riot_web_enabled|bool and matrix_riot_web_self_build"
 
 - name: Ensure Matrix riot-web configuration installed
   copy: