split: allow writing to symlink to nonseekable

[oech3]

Fixes #13409

[codspeed-hq]

Merging this PR will improve performance by 7.79%

⚡ 1 improved benchmark
✅ 338 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	complex_relative_date	147.8 µs	137.2 µs	+7.79%

Tip

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_{Comparing oech3:split-stream (f9be24f) with main (d7a2e2f)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/isatty (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)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.