Update static macOS Nix build to reflect fixed build process on Nixpkgs

[Thesola10]

I need to look into and integrate NixOS/nixpkgs#235990 with Nixie's static Nix binary source to simplify derivations and make them more easily maintainable.

[Thesola10]

NixOS/nixpkgs#180771 appears to prevent us from easily shipping a prebuilt static Nix for x86 macOS. This still allows us to mostly stop using Bitrise, as it will only be required for Universal fakedir. Ideally github/roadmap#528 would allow us to migrate that to GitHub Actions.