uniq: honor stdbuf mode

[victor-prokhorov]

fixes stdbuf #10390
main goal is to make gnu stdbuf test pass, i believe the issue is in the uniq

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[Ecordonnier]

See #7967 for some context

[victor-prokhorov]

thanks for the pointer to the issue @Ecordonnier, i went down that rabbit hole and tried to poc switchable buffering based on rust-lang/rust#78515. closing this until something lands on the rust side.

[codspeed-hq]

Merging this PR will degrade performance by 3.94%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 296 untouched benchmarks
⏩ 46 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	uniq_heavy_duplicates[10000]	2 ms	2.1 ms	-3.94%
⚡	Memory	uniq_case_insensitive[10000]	141.9 KB	16.6 KB	×8.6
⚡	Memory	uniq_heavy_duplicates[10000]	188.7 KB	63.3 KB	×3
⚡	Memory	uniq_with_count[10000]	141.9 KB	16.6 KB	×8.6

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_{Comparing victor-prokhorov:fix-stdbuf-test-by-fixing-uniq (64762d3) with main (c79e686)}

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. ↩