[codex] re-enable absolute workdir integration test (#28581)

## Why

In #28146 I missed the invariant that an absolute `exec_command` workdir
must override the environment cwd. The existing integration test would
have caught that regression, but it was ignored as flaky.

## What

Re-enable `unified_exec_respects_workdir_override`.

## Validation

`just test -p codex-core unified_exec_respects_workdir_override`

codex-rs/core/tests/suite/unified_exec.rs

@@ -521,7 +521,6 @@ async fn unified_exec_resolves_relative_workdir() -> Result<()> {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-#[ignore = "flaky"]
 async fn unified_exec_respects_workdir_override() -> Result<()> {
     skip_if_no_network!(Ok(()));
     skip_if_sandbox!(Ok(()));