support specifiying singing key in variable

roles/synapse/tasks/config.yml

@@ -1,5 +1,14 @@
 ---
 
+- name: Ensure signing key matches matrix_signing_key variable
+  ansible.builtin.copy:
+    content: "{{ matrix_signing_key }}"
+    dest: /etc/matrix-synapse/homeserver.signing.key
+    owner: matrix-synapse
+    group: nogroup
+    mode: 0600
+  when: matrix_signing_key is defined
+
 - name: Ensure signing key permissions set correctly
   ansible.builtin.file:
     path: /etc/matrix-synapse/homeserver.signing.key
@@ -7,6 +16,7 @@
     owner: matrix-synapse
     group: nogroup
     mode: 0600
+  when: matrix_signing_key is not defined
 
 - name: Ensure synapse configs are in place
   ansible.builtin.template: