Do not run matrix-nginx-proxy config injection tasks unless necessary

These `init.yml` (now `inject_into_nginx_proxy.yml`) tasks do not need
to `always` run. They only need to run for `setup-all` and
`setup-nginx-proxy`. Unless we're dealing with these 2 tags, we can
spare ourselves a lot of work.

This patch also moves the `when` statement from `init.yml` into
`main.yml` in an effort to further optimize things by potentially
avoiding the extra file include.

roles/custom/matrix-bot-maubot/tasks/main.yml

@@ -1,8 +1,10 @@
 ---
 
-- ansible.builtin.import_tasks: "{{ role_path }}/tasks/init.yml"
+- when: matrix_bot_maubot_enabled | bool
+  ansible.builtin.import_tasks: "{{ role_path }}/tasks/inject_into_nginx_proxy.yml"
   tags:
-    - always
+    - setup-all
+    - setup-nginx-proxy
 
 - ansible.builtin.import_tasks: "{{ role_path }}/tasks/validate_config.yml"
   when: "run_setup|bool and matrix_bot_maubot_enabled|bool"