fix(nix): use correct version from Cargo.toml in flake build (#11770)

## Summary

- When building via `nix build`, the binary reports `codex-cli 0.0.0`
because the workspace `Cargo.toml` uses `0.0.0` as a placeholder on
`main`. This causes the update checker to always prompt users to upgrade
even when running the latest code.
- Reads the version from `codex-rs/Cargo.toml` at flake evaluation time
using `builtins.fromTOML` and patches it into the workspace `Cargo.toml`
before cargo builds via `postPatch`.
- On release commits (e.g. tag `rust-v0.101.0`), the real version is
used as-is. On `main` branch builds, falls back to
`0.0.0-dev+<shortRev>` (or `0.0.0-dev+dirty`), which the update
checker's `parse_version` ignores — suppressing the spurious upgrade
prompt.

| Scenario | Cargo.toml version | Nix `version` | Binary reports |
Upgrade nag? |
|---|---|---|---|---|
| Release commit (e.g. `rust-v0.101.0`) | `0.101.0` | `0.101.0` |
`codex-cli 0.101.0` | Only if newer exists |
| Main branch (committed) | `0.0.0` | `0.0.0-dev+b934ffc` | `codex-cli
0.0.0-dev+b934ffc` | No |
| Main branch (uncommitted) | `0.0.0` | `0.0.0-dev+dirty` | `codex-cli
0.0.0-dev+dirty` | No |

## Test plan

- [ ] `nix build` from `main` branch and verify `codex --version`
reports `0.0.0-dev+<shortRev>` instead of `0.0.0`
- [ ] Verify the update checker does not show a spurious upgrade prompt
for dev builds
- [ ] Confirm that on a release commit where `Cargo.toml` has a real
version, the binary reports that version correctly

flake.nix

@@ -9,7 +9,7 @@
     };
   };
 
-  outputs = { nixpkgs, rust-overlay, ... }:
+  outputs = { self, nixpkgs, rust-overlay, ... }:
     let
       systems = [
         "x86_64-linux"
@@ -18,6 +18,19 @@
         "aarch64-darwin"
       ];
       forAllSystems = f: nixpkgs.lib.genAttrs systems f;
+
+      # Read the version from the workspace Cargo.toml (the single source of
+      # truth used by the release workflow).
+      cargoToml = builtins.fromTOML (builtins.readFile ./codex-rs/Cargo.toml);
+      cargoVersion = cargoToml.workspace.package.version;
+
+      # When building from a release commit the Cargo.toml already carries the
+      # real version (e.g. "0.101.0").  On the main branch it is the placeholder
+      # "0.0.0", so we fall back to a dev version derived from the flake source.
+      version =
+        if cargoVersion != "0.0.0"
+        then cargoVersion
+        else "0.0.0-dev+${self.shortRev or "dirty"}";
     in
     {
       packages = forAllSystems (system:
@@ -27,6 +40,7 @@
             overlays = [ rust-overlay.overlays.default ];
           };
           codex-rs = pkgs.callPackage ./codex-rs {
+            inherit version;
             rustPlatform = pkgs.makeRustPlatform {
               cargo = pkgs.rust-bin.stable.latest.minimal;
               rustc = pkgs.rust-bin.stable.latest.minimal;