config: init: fix crash in mirrors prompt

This was missed in fc010bc7

Before this fix:

...

[18:34:35] Enable background timer to prevent repeated sudo authorization? (y/n) [n]: y
(020272) [18:34:44] ERROR: 'Config' object has no attribute 'mirrors_postmarketos'
(020272) [18:34:44] See also: <https://postmarketos.org/troubleshooting>
(020272) [18:34:44] Traceback (most recent call last):
  File "/home/arnavion/src/pmbootstrap/pmb/__init__.py", line 72, in main
    return config_init.frontend(args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/arnavion/src/pmbootstrap/pmb/config/init.py", line 729, in frontend
    ask_for_additional_options(config)
  File "/home/arnavion/src/pmbootstrap/pmb/config/init.py", line 542, in ask_for_additional_options
    logging.info("Selected mirror:" f" {','.join(context.config.mirrors_postmarketos)}")
                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/arnavion/src/pmbootstrap/pmb/core/config.py", line 130, in __getattribute__
    return super().__getattribute__(key)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Config' object has no attribute 'mirrors_postmarketos'

Fix #2405 (closed)