Add Darwin Nix flake to Allow import within nix-darwin

[dzmitry-lahoda]

Note

AI generated, I just promoted to use idiomatic approaches with minimal layer of adhoc fixes.

Summary

• add an aarch64-darwin-only Nix flake for omlx
• read the package version from omlx/_version.py
• run the installed omlx --help during the Nix install check

Verification

• nix build .# --no-link --no-write-lock-file
• nix run .# -- --help
• nix eval .#packages.aarch64-darwin.default.version --raw --no-write-lock-file