ci: Use Nix based image for pre-commit

[mathbunnyru]

High Level Overview of Change

This is a last usage of old images.
Using nix-nixos flavor, because it has nix installed and thus would format Nix files too.

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)