I apologize for my ignorance on nix.shell as I am a flake user, but I’ll try to help out anyway.
Personally I would highly recommend avoiding global installs as much as possible, as it makes picking up a project 2 or 3 years down the line harder, and instead using flakes + nix-direnv, which automatically loads the packages you need for a particular project whenever you cd into its directory.
You can see an example of this in my Advent of Code repo, but I will also post a minimal flake.nix that should work for your use case.
{
description = "A basic Zig flake";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
flake-parts.url = "github:hercules-ci/flake-parts";
};
outputs =
inputs@{ self
, flake-parts
, nixpkgs
, nixpkgs-unstable
}:
flake-parts.lib.mkFlake { inherit inputs; } {
imports = [ ];
systems = [ "x86_64-linux" ]; # Adjust according to your needs
perSystem =
{ pkgs, system, inputs', ... }:
{
# Allow unstable and Unfree packages
_module.args.pkgs = import nixpkgs {
inherit system;
config.allowUnfree = true;
overlays = [
(final: prev: { unstable = inputs'.nixpkgs-unstable.legacyPackages; })
];
};
devShells.default = pkgs.mkShell
{
buildInputs = with pkgs; [
unstable.zls
unstable.zig
# Zed may or may not work if installed globally,
# With a gloabl VSCode install you can get the direnv extension
# to auto load your dev shell.
#
# commandline editors like neovim and helix automatically
# work ofc with direnv
zed-editor
];
};
};
};
};
}
I realize this probably isn’t the answer you’re looking for, but I would argue that the power of nix is its reproducability, and the more global (system level) state you have, the more you poison that reproducability. Even though all my projects use this setup, I still have been shot in the foot with a global nushell install as my default shell, which can result in different shell versions being used depending on how I run the script 
Another benefit of doing things this way is that if you manage to bundle your code as a flake pkg, (which can be a major PITA, I know), and you experience build issues you need help with, any (flakes enabled) nix user can easily check if your build works by running nix run 'github:user/repo' on their machine.
Sorry if this is all obvious and I’m being a jerk here.