Prefer just test over cargo test in docs (#23910)

`cargo test` for the core and other crates fails on a fresh macOS
checkout without the right stack size variable. This change encourages
using the just test command that sets the environment up correctly.

As a bonus, this should encourage agents to get more benefit out of
nextest's parallel execution.

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

@@ -2,7 +2,8 @@
 
 //! Optional smoke tests that hit the real OpenAI /v1/responses endpoint. They are `#[ignore]` by
 //! default so CI stays deterministic and free. Developers can run them locally with
-//! `cargo test --test live_cli -- --ignored` provided they set a valid `OPENAI_API_KEY`.
+//! `just test -p codex-core --test all --run-ignored only live_cli` provided they set a valid
+//! `OPENAI_API_KEY`.
 
 use assert_cmd::prelude::*;
 use predicates::prelude::*;