Fix self-building for synapse-admin

roles/matrix-synapse-admin/tasks/setup.yml

@@ -24,7 +24,7 @@
   docker_image:
     name: "{{ matrix_synapse_admin_docker_image }}"
     source: build
-    force_source: "{{ matrix_synapse_admin_git_pull_results }}"
+    force_source: "{{ matrix_synapse_admin_git_pull_results.changed }}"
     build:
       dockerfile: Dockerfile
       path: "{{ matrix_synapse_admin_docker_src_files_path }}"