fix: resolve Jinja2 .keys ambiguity, fastfetch availability, and python interpreter

- Use bracket notation item['keys'] instead of item.keys to avoid
  conflict with Python dict .keys() method
- Remove fastfetch from Debian 12 package list (only available in 13+)
- Set explicit python interpreter path for post-reboot tasks

roles/virtualization/templates/cloud-user-data.yml.j2

@@ -11,7 +11,7 @@ users:
     sudo: "{{ user.sudo | default('ALL=(ALL) NOPASSWD:ALL') }}"
     passwd: "{{ user.password | password_hash('sha512') }}"
     lock_passwd: false
-{% set ssh_keys = user.keys | default([]) %}
+{% set ssh_keys = user['keys'] | default([]) %}
 {% if ssh_keys | length > 0 %}
     ssh_authorized_keys:
 {% for key in ssh_keys %}