As of Catalina, there's no sane way to install the Nix package manager without losing functionality because macOS now disallows creating new files in the root directory[1]. Nix stores its packages in the /nix directory and it's not possible to migrate without causing major disruptions for existing NixOS and other Linux users. This is too bad, since apart from Nix being a nice package manager, it also provides a sane binary package for Emacs. The Homebrew core/cask versions only provides a limited feature set[2][3].
[1]: https://github.com/NixOS/nix/issues/2925
[2]: https://github.com/Homebrew/homebrew-core/issues/31510
[3]: https://github.com/caldwell/build-emacs/search?q=support+is%...