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.lock

@@ -1,30 +1,12 @@
 {
   "nodes": {
-    "flake-utils": {
-      "inputs": {
-        "systems": "systems"
-      },
-      "locked": {
-        "lastModified": 1731533236,
-        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1744463964,
-        "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
+        "lastModified": 1758427187,
+        "narHash": "sha256-pHpxZ/IyCwoTQPtFIAG2QaxuSm8jWzrzBGjwQZIttJc=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
+        "rev": "554be6495561ff07b6c724047bdd7e0716aa7b46",
         "type": "github"
       },
       "original": {
@@ -36,44 +18,7 @@
     },
     "root": {
       "inputs": {
-        "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs",
-        "rust-overlay": "rust-overlay"
-      }
-    },
-    "rust-overlay": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1746844454,
-        "narHash": "sha256-GcUWDQUDRYrD34ol90KGUpjbVcOfUNbv0s955jPecko=",
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "rev": "be092436d4c0c303b654e4007453b69c0e33009e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "type": "github"
-      }
-    },
-    "systems": {
-      "locked": {
-        "lastModified": 1681028828,
-        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
-        "owner": "nix-systems",
-        "repo": "default",
-        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-systems",
-        "repo": "default",
-        "type": "github"
+        "nixpkgs": "nixpkgs"
       }
     }
   },