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>

codex-rs/default.nix

@@ -1,42 +1,31 @@
-{ pkgs, monorep-deps ? [], ... }:
-let
+{
+  openssl,
+  rustPlatform,
+  pkg-config,
+  lib,
+  ...
+}:
+rustPlatform.buildRustPackage (finalAttrs: {
   env = {
-    PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig:$PKG_CONFIG_PATH";
+    PKG_CONFIG_PATH = "${openssl.dev}/lib/pkgconfig:$PKG_CONFIG_PATH";
   };
-in
-rec {
-  package = pkgs.rustPlatform.buildRustPackage {
-    inherit env;
-    pname = "codex-rs";
-    version = "0.1.0";
-    cargoLock.lockFile = ./Cargo.lock;
-    doCheck = false;
-    src = ./.;
-    nativeBuildInputs = with pkgs; [
-      pkg-config
-      openssl
-    ];
-    meta = with pkgs.lib; {
-      description = "OpenAI Codex command‑line interface rust implementation";
-      license = licenses.asl20;
-      homepage = "https://github.com/openai/codex";
-    };
+  pname = "codex-rs";
+  version = "0.1.0";
+  cargoLock.lockFile = ./Cargo.lock;
+  doCheck = false;
+  src = ./.;
+  nativeBuildInputs = [
+    pkg-config
+    openssl
+  ];
+
+  cargoLock.outputHashes = {
+    "ratatui-0.29.0" = "sha256-HBvT5c8GsiCxMffNjJGLmHnvG77A6cqEL+1ARurBXho=";
   };
-  devShell = pkgs.mkShell {
-    inherit env;
-    name = "codex-rs-dev";
-    packages = monorep-deps ++ [
-      pkgs.cargo
-      package
-    ];
-    shellHook = ''
-      echo "Entering development shell for codex-rs"
-      alias codex="cd ${package.src}/tui; cargo run; cd -"
-      ${pkgs.rustPlatform.cargoSetupHook}
-    '';
+
+  meta = with lib; {
+    description = "OpenAI Codex command‑line interface rust implementation";
+    license = licenses.asl20;
+    homepage = "https://github.com/openai/codex";
   };
-  app = {
-    type = "app";
-    program = "${package}/bin/codex";
-  };
-}
+})