feat: add flake.nix for dockerless setup

[Azteczek]

Description

This PR adds a feature for NixOS users, it adds a flake file (flake.nix and flake.lock) which can replace the need for installing docker.

Flakes is an experimental feature of the Nix package manager.
What flakes can do:

• It lets you download something that isn't available in https://search.nixos.org/packages
• Llets you have reproducible environments.

There won't be any need to change anything (only maybe adding a part in readme.md), just to add these 2 files.

Notes for Reviewers

If my description of flakes doesn't make sense, look at:

• Official wiki: https://nixos.wiki/wiki/Flakes
• Reference implementation: https://github.com/Azteczek/LocalAI-On-NixOS (readme explains how it works)

Signed commits

• Yes, I signed my commits.

[mudler]

adding @richiejp for reviewing as he is already a NixOS user

[richiejp]

Great stuff, just one question about the Unsloth link?