Fix nix build

[lukasl-dev]

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:

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.
  ...

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[lukasl-dev]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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[etraut-openai]

Thanks for the contribution!