Run package updates outside the project on non-Windows platforms (#58403)

## Why

Package managers search parent directories for project configuration and hooks, which can influence Codex updates launched from a project directory.

## What changed

Run non-Windows update commands with `/` as their working directory.

## Testing

Add a regression test using a fake `pnpm` executable to verify that updates run outside the project even when `TMPDIR` points into it.

GitOrigin-RevId: a39b36872a62f2404aadd0614e1486d0755636fc

codex-rs/cli/src/main.rs

@@ -848,6 +848,8 @@ fn run_update_action(
                 .collect();
             std::process::Command::new(&command_path)
                 .args(&normalized_args)
+                // Package managers search parent directories for project config and hooks.
+                .current_dir("/")
                 .status()?
         }
     };