feat: add Nix CI

[blackheaven]

Description

Add full Nix build and associated CI.

Motivation and Context

Following my previous Nix PR, I was suggested to add the CI part.

It had took a full Nix build support (instead of just an environment definition.

How Has This Been Tested?

Locally via nix flake check -Lv.

Screenshots / Logs (if applicable)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.97%. Comparing base (0fabf22) to head (071044d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #939      +/-   ##
==========================================
+ Coverage   42.91%   42.97%   +0.06%     
==========================================
  Files          21       21              
  Lines        1713     1713              
==========================================
+ Hits          735      736       +1     
+ Misses        978      977       -1     

Flag	Coverage Δ
unit-tests	42.97% <ø> (+0.06%)	⬆️

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[orhun]

Awesome, thank you!

[blackheaven]

Thanks for your guidance!