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!