restructure flake for codex-rs (#888)

Right now since the repo is having two different implementations of
codex, flake was updated to work with both typescript implementation and
rust implementation

flake.lock

@@ -37,7 +37,28 @@
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs"
+        "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": {