Fix nix build (#4048)

I dropped the build of the old cli from the flake, where the default.nix
already seemed to removed in a previous iterations. Then I updated
flake.nix and codex-rs expression to be able to build again (see
individual commits for details).

Tested by running the following builds:


```
$ nix build .#packages.x86_64-linux.codex-rs
$ nix build .#packages.aarch64-darwin.codex-cli
```

---------

Signed-off-by: Jörg Thalheim <joerg@thalheim.io>

flake.nix

@@ -1,57 +1,28 @@
 {
   description = "Development Nix flake for OpenAI Codex CLI";
 
-  inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    flake-utils.url = "github:numtide/flake-utils";
-    rust-overlay = {
-      url = "github:oxalica/rust-overlay";
-      inputs.nixpkgs.follows = "nixpkgs";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { nixpkgs, ... }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems f;
+    in
+    {
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          codex-rs = pkgs.callPackage ./codex-rs { };
+        in
+        {
+          codex-rs = codex-rs;
+          default = codex-rs;
+        }
+      );
     };
-  };
-
-  outputs = { nixpkgs, flake-utils, rust-overlay, ... }: 
-    flake-utils.lib.eachDefaultSystem (system:
-      let
-        pkgs = import nixpkgs {
-          inherit system;
-        };
-        pkgsWithRust = import nixpkgs {
-          inherit system;
-          overlays = [ rust-overlay.overlays.default ];
-        };
-        monorepo-deps = with pkgs; [
-          # for precommit hook
-          pnpm
-          husky
-        ];
-        codex-cli = import ./codex-cli {
-          inherit pkgs monorepo-deps;
-        };
-        codex-rs = import ./codex-rs {
-          pkgs = pkgsWithRust;
-          inherit monorepo-deps;
-        };
-      in
-      rec {
-        packages = {
-          codex-cli = codex-cli.package;
-          codex-rs = codex-rs.package;
-        };
-
-        devShells = {
-          codex-cli = codex-cli.devShell;
-          codex-rs = codex-rs.devShell;
-        };
-
-        apps = {
-          codex-cli = codex-cli.app;
-          codex-rs = codex-rs.app;
-        };
-
-        defaultPackage = packages.codex-cli;
-        defaultApp = apps.codex-cli;
-        defaultDevShell = devShells.codex-cli;
-      }
-    );
 }