Add macOS (Darwin) configuration build job to CI

[Meatwo310]

Summary

This PR adds a new CI job to build and test macOS (Darwin) NixOS configurations, complementing the existing Linux NixOS build job.

Key Changes

• Added build-darwin job to the GitHub Actions workflow that:
  • Runs on macos-latest runners
  • Builds the m2air Darwin configuration
  • Uses the same Nix tooling and caching infrastructure (Cachix) as the Linux builds
  • Depends on the check job to ensure it runs after linting/validation passes
  • Follows the same build pattern as the existing build-linux job but targets darwinConfigurations instead of nixosConfigurations

Implementation Details

• The job uses fail-fast: false to allow other matrix builds to complete even if one fails
• Configured with the same Nix and Cachix actions as the Linux build job for consistency
• Uses the meatwo310-dotnix Cachix cache for build artifact caching
• Accepts Nix flake configuration via accept-flake-config = true

https://claude.ai/code/session_01UESBvZnuwaS8WsQpRY2Vi5