Fix nix build (openai#6230)

Previously, the `nix build .#default` command fails due to a missing
output hash in the `./codex-rs/default.nix` for `crossterm-0.28.1`:

```
error: No hash was found while vendoring the git dependency crossterm-0.28.1. You can add
a hash through the `outputHashes` argument of `importCargoLock`:

outputHashes = {
 "crossterm-0.28.1" = "<hash>";
};

If you use `buildRustPackage`, you can add this attribute to the `cargoLock`
attribute set.
```

This PR adds the missing hash:

```diff
cargoLock.outputHashes = {
  "ratatui-0.29.0" = "sha256-HBvT5c8GsiCxMffNjJGLmHnvG77A6cqEL+1ARurBXho=";
+ "crossterm-0.28.1" = "sha256-6qCtfSMuXACKFb9ATID39XyFDIEMFDmbx6SSmNe+728=";
};
```

With this change, `nix build .#default` succeeds:

```
> nix build .#default --max-jobs 1 --cores 2

warning: Git tree '/home/lukas/r/github.com/lukasl-dev/codex' is dirty
[1/0/1 built] building codex-rs-0.1.0 (buildPhase)[1/0/1 built] building codex-rs-0.1.0 (buildP[1/0/1 built] building codex-rs-0.1.0 (buildPhase):    [1/0/1 built] building codex-rs-0.1.0 (b[1/0/1 built] building codex-rs-0.1.0 (buildPhase):    Compi[1/0/1 built] building codex-rs-0.1

> ./result/bin/codex
  You are running Codex in /home/lukas/r/github.com/lukasl-dev/codex

  Since this folder is version controlled, you may wish to allow Codex to work in this folder without asking for approval.
  ...
```

Author: lukasl-dev

codex-rs/default.nix

@@ -5,7 +5,7 @@
   lib,
   ...
 }:
-rustPlatform.buildRustPackage (finalAttrs: {
+rustPlatform.buildRustPackage (_: {
   env = {
     PKG_CONFIG_PATH = "${openssl.dev}/lib/pkgconfig:$PKG_CONFIG_PATH";
   };
@@ -21,6 +21,7 @@ rustPlatform.buildRustPackage (finalAttrs: {
 
   cargoLock.outputHashes = {
     "ratatui-0.29.0" = "sha256-HBvT5c8GsiCxMffNjJGLmHnvG77A6cqEL+1ARurBXho=";
+    "crossterm-0.28.1" = "sha256-6qCtfSMuXACKFb9ATID39XyFDIEMFDmbx6SSmNe+728=";
   };
 
   meta = with lib; {