Switch to flakes based configuration

[TanvirOnGH]

Description

Switch to flake-based configuration for improved maintainability, reproducibility, and compatibility with NixOS best practice.

Motivation

This change is essential to modernize our configuration approach and align with the evolving standards within the NixOS community. Flakes provide a more declarative and version-controlled way to manage system configurations, making it easier to reproduce environments and collaborate on system configurations.

Early attempts

• https://github.com/TanvirOnGH/nixos-config/pull/2

• https://github.com/NiXium-org/NiXium/pull/84#issuecomment-2374196149

[TanvirOnGH]

Things to explore next: hive (or flake-parts + haumea | omnibus) configuration.