refactor: replace rustix::pipe with std::io::pipe

[xtqqczze]

std::io::pipe provides PipeReader and PipeWriter types, wrappers around the underlying pipe file descriptors/handles that implement the Read and Write traits respectively. This removes the direct rustix dependency while preserving existing behavior.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[oech3]

I noticed that pipe syscall was replaced by pipe2. But it should not hurt.

[oech3]

I agree with some names of vars are not ideal, but I don't think we need to hurry generating many confliction.
The main purpose of this PR should be migration to std.

[xtqqczze]

@oech3 Sorry, I didn’t realize that renaming the variables in 44b9416 would introduce conflicts. I’ve dropped that commit now.

[oech3]

Thanks.

[oech3]

error: unused import: std::fs::File

[oech3]

Any std replacement for https://doc.servo.org/rustix/pipe/constant.PIPE_BUF.html ?

[xtqqczze]

• CICD / Build (macos-latest, x86_64-apple-darwin, feat_os_unix, true) (pull_request) failure tracked by test_terminate_child_on_receiving_terminate intermittent failure #11870.

[cakebaker]

Thanks to both of you!