This command upgrades Nix to the stable version declared in Nixpkgs.
This stable version is defined in [nix-fallback-paths.nix](https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix)
and updated manually. It may not always be the latest tagged release.
By default, it locates the directory containing the `nix` binary in the `$PATH`