yes: don't mix buffered write and zero-copy

[oech3]

No need to double buffering.
Buffered write after zero-copy does not cause any breakage. But it is a risk of changing order of outputs when tried to improve fallback in the future (I'm planning to do that) by mixing them.

[github-actions]

GNU testsuite comparison:

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

[codspeed-hq]

Merging this PR will not alter performance

✅ 317 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:yes-raw (9db35b0) with main (546e233)²}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (645c1e3) during the generation of this report, so 546e233 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩