buf_copy/linux.rs: replace flush by raw write

[oech3]

flush is just an workaround to avoid wrong order of output. Proper solution is writing to fd without buffering.
(This matches with GNU cp's unbuffered cp /dev/stdin ....)

Closes #12405

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[xtqqczze]

@oech3 Sorry that #12405 overlaps, if I had noticed earlier, I would have suggested the changes here instead. That said, I think that PR takes the better approach by using types to prevent incorrect output ordering.

[oech3]

Stream does NOT mean it is pipe. Please remove PipeReader.

[xtqqczze]

Stream does NOT mean it is pipe. Please remove PipeReader.

coreutils/src/uucore/src/lib/features/buf_copy.rs

Line 59 in 157bbab

copy_stream(&mut pipe_read, &mut dest_file).unwrap();

[oech3]

Well, if our cp /dev/stdin is lacking support for non-pipe stream, we should support it too. No benefit to restrict. cc: reviers Please merge this and ignore another one.