macOS nix-static builder overhaul

[Thesola10]

This PR is meant for testing and adapting NixOS/nixpkgs#346043 to Nixie's static-bins package, in an effort to leverage as much code from upstream as possible and make updates easier.

Since 3f8326b, this PR also overhauls the static Nix build system to directly use NixOS/nix rather than Nixpkgs, with possible patches per-platform.

[Thesola10]

Once this change hits master on Nixpkgs, and tests confirm the resulting executables are quasi-static, this will be the new build system for Nixie, allowing for much easier and more frequent updates with macOS.

[Thesola10]

New approach based on upstream Nix build system yields very promising results:

$ llvm-otool -L result/bin/nix
result/bin/nix:
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.0.0)
	/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1348.28.0)
	/System/Library/Frameworks/SystemConfiguration.framework/Versions/A/SystemConfiguration (compatibility version 1.0.0, current version 888.30.2)
	/System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 57740.31.2)

however the libfetchers component appears to be broken, preventing anything flake-related from working:

error: input 'flake://nixpkgs' is unsupported

Getting close!