pipes.rs: deduplicate some code

[oech3]

Help #12387 and some my PRs.

[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/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[oech3]

Hmm... rustix::io::read does not work

[oech3]

I considered that std::io::copy breaks fuse, But input is actually pipe... Also We have Raw{Reader,Writer} to remove buffering from io::copy.