Use codex-linux-sandbox in unified exec (#6480)

Unified exec isn't working on Linux because we don't provide the correct
arg0.

The library we use for pty management doesn't allow setting arg0
separately from executable. Use the same aliasing strategy we use for
`apply_patch` for `codex-linux-sandbox`.

Use `#[ctor]` hack to dispatch codex-linux-sandbox calls.


Addresses https://github.com/openai/codex/issues/6450

codex-rs/Cargo.lock

@@ -1067,6 +1067,7 @@ dependencies = [
  "chrono",
  "codex-app-server-protocol",
  "codex-apply-patch",
+ "codex-arg0",
  "codex-async-utils",
  "codex-file-search",
  "codex-git",
@@ -1081,6 +1082,7 @@ dependencies = [
  "codex-windows-sandbox",
  "core-foundation 0.9.4",
  "core_test_support",
+ "ctor 0.5.0",
  "dirs",
  "dunce",
  "env-flags",