head: fix error message when input is a directory

[aguimaraes]

Fixes #11485

head . incorrectly reports:

head: error writing 'standard output': Is a directory

It should report:

head: error reading '.': Is a directory

The cause: io::copy can fail from either the read or write side,but the error was always attributed to stdout via wrap_in_stdout_error.

[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/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/pr/bounded-memory is now passing!
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!

[cakebaker]

The test currently fails on Windows, you probably have to gate it.

[aguimaraes]

The test currently fails on Windows, you probably have to gate it.

I will try to move the directory check before File::open, using std::path::Path::is_dir(), it should work on Windows too, wdyt?

[codspeed-hq]

Merging this PR will degrade performance by 4.8%

❌ 1 regressed benchmark
✅ 303 untouched benchmarks
⏩ 46 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cp_large_file[16]	265.1 µs	278.4 µs	-4.8%

---

_{Comparing aguimaraes:head-fix-directory-error-message (abbde9e) with main (aab0dda)}

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:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!