stdbuf: point the caret at the failing part of a buffering mode

[sylvestre]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
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/seq/seq-epipe is now being skipped but was previously passing.
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 57 untouched benchmarks
⏩ 350 skipped benchmarks¹

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_{Comparing caret-stdbuf (8360717) with caret-shred (973de53)²}

Footnotes

1. 350 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 caret-shred (f2987b5) during the generation of this report, so f790b87 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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