[cp] cp::test_cp_fail_perm test fails

[jgarzik]

On Ubuntu 22.04.4 LTS, cargo test fails test cp::test_cp_fail_perm. The test passes on CI-macos, CI-linux and my local MacOS test env.

---- cp::test_cp_fail_perm stdout ----
thread 'cp::test_cp_fail_perm' panicked at plib/src/testing.rs:68:5:
assertion `left == right` failed
  left: "cp: Permission denied\n"
 right: "cp: cannot open '/home/jgarzik/repo/posixutils-rs/target/tmp/test_cp_fail_perm/D/a' for reading: Permission denied\n"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    cp::test_cp_fail_perm

test result: FAILED. 104 passed; 1 failed; 6 ignored; 0 measured; 0 filtered out; finished in 3.10s