ref(system): Fix ssh authorized keys task

Fixed the loop for authorized keys. While I read previously that the
Ansible module can take keys in the array format:

```yaml
    key:
      - key1
      - key2
      - ...
```

This seems to not be the case.
Instead, we now do a 'sub-loop' through all the existing authorized_keys
entries in the data structure, running the task once for each key.

This also means we can simplify the 'when' condition to only check the
data structure itself exists, not the key since we only loop once for
each existing key anyway.

More in-depth explanation on the subelements filter here:
https://docs.ansible.com/projects/ansible/latest/playbook_guide/playbooks_filters.html#combining-objects-and-subelements

Concise explanation of use here:
https://overflow.ducks.party/questions/56086290/how-can-i-traverse-nested-lists-in-ansible

One drawback:
we can now _not_ change the key setting in the module to be exclusive
(`exclusive: true` for `authorized_keys` module). As described in the
documentation, if there are more than one key for a user, this would
lead to the following keys overwriting the first key.

Currently do not know how to fix this, but we are not supplying
exclusive keys so it is fine for the moment.

roles/system/tasks/main.yaml

@@ -70,11 +70,11 @@
 
 - name: Add authorized SSH keys
   ansible.posix.authorized_key:
-    user: "{{ item.name }}"
+    user: "{{ item.0.name }}"
     state: present
-    key: "{{ item.authorized_keys }}"
-  loop: "{{ system_users }}"
-  when: system_users is defined and item.authorized_keys is defined
+    key: "{{ item.1 }}"
+  loop: "{{ system_users | subelements('authorized_keys', skip_missing=True) }}"
+  when: system_users is defined
   tags:
     - ssh
   become: true