Move self-building git repository URLs to variables (stop hardcoding)

roles/matrix-client-element/tasks/setup.yml

@@ -26,7 +26,7 @@
 
 - name: Ensure Element repository is present on self-build
   git:
-    repo: https://github.com/vector-im/riot-web.git
+    repo: "{{ matrix_client_element_container_image_self_build_repo }}"
     dest: "{{ matrix_client_element_docker_src_files_path }}"
     version: "{{ matrix_client_element_docker_image.split(':')[1] }}"
     force: "yes"