pipes.rs: don't mix raw write and buffered write

[oech3]

This fallback code pass is rarely used, but line-buffered which should be removed as same as 055a66b .

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!

[cakebaker]

Thanks!