When attempting to follow the docs on the installer, this error comes
up:
```
$ clan flash --flake git+https://git.clan.lol/clan/clan-core \
--ssh-pubkey ~/.ssh/id_ed25519.pub \
--keymap us \
--language en_US.utf-8 \
--dry-run flash-installer
ERROR: Language 'en_US.utf-8' is not a valid language. Run 'clan flash --list-languages' to see a list of possible languages.
```
I only decided to clean up the docs, but I'm also okay with just
downcasing the locales in `list_possible_languages()`.
Injecting nixos configuration and potentially overriding settings a user
made and can cause surprises.
In most cases, users want to just make these option part of their NixOS
configuration and by having the rest in the command line
we make it more explicit what other configuration is being applied.
