test_stdbuf_no_fork_regression intermittent failure

---- test_stdbuf::test_stdbuf_no_fork_regression stdout ----
bin: "/target/i686-unknown-linux-gnu/debug/coreutils"

thread 'test_stdbuf::test_stdbuf_no_fork_regression' panicked at tests/by-util/test_stdbuf.rs:408:5:
Expected 'sleep' command at PID 34599, got: /target/i686-unknown-linux-gnu/d
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: tests::test_stdbuf::test_stdbuf_no_fork_regression
             at ./tests/by-util/test_stdbuf.rs:408:5
   3: tests::test_stdbuf::test_stdbuf_no_fork_regression::{{closure}}
             at ./tests/by-util/test_stdbuf.rs:351:36
   4: core::ops::function::FnOnce::call_once
             at /rust/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

https://github.com/uutils/coreutils/actions/runs/30183522080/job/89744406150?pr=12317

Start with tests/by-util/test_stdbuf.rs, especially test_stdbuf_no_fork_regression around line 408, and review the linked CI run for the intermittent failure. Reproduce the test and trace why the reported PID is not the expected sleep command. Done means the regression test reliably identifies the expected process without intermittent failures.