stdbuf: harden no-fork cmdline polling

[puneetdixit200]

Summary:

• make the no-fork regression test wait until /proc cmdline resolves to the expected sleep target
• add coverage for the transient /target/x86_64-unknown-linux-gnu cmdline value observed in CI
• keep the change scoped to the stdbuf test polling logic

Tests:

• CARGO_HOME=/private/tmp/uutils-stdbuf-cargo-home CARGO_TARGET_DIR=/private/tmp/uutils-stdbuf-target cargo test -q --features stdbuf,test_risky_names --no-default-features test_stdbuf_cmdline_poll_ignores_transient_target_prefix
• docker run --rm ... rust:1.88-bookworm cargo test -q --features stdbuf,test_risky_names --no-default-features test_stdbuf (14 passed)
• CARGO_HOME=/private/tmp/uutils-stdbuf-cargo-home CARGO_TARGET_DIR=/private/tmp/uutils-stdbuf-target cargo clippy -q --features stdbuf,test_risky_names --no-default-features --test tests -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.