nginx: clarify intent regarding check mode

nginx/tasks/munin_vhost.yml

@@ -30,7 +30,10 @@
   ansible.builtin.copy:
     src: systemd/spawn-fcgi-munin-graph.service
     dest: /etc/systemd/system/spawn-fcgi-munin-graph.service
+    force: yes
 
+# WARN: there is no (apparent) way to check if the service exists
+#       so we disable this task in check mode.
 - name: Systemd unit for Munin-fcgi is started
   ansible.builtin.systemd:
     name: spawn-fcgi-munin-graph