fuzz: replace unsafe libc::dup by rustix::io::dup

[blixygetir]

Fixes #12110

Replaces unsafe libc::dup calls with the safe rustix::io::dup wrapper in fuzz/uufuzz/src/lib.rs. The code now uses Result instead of checking for -1, and leverages IntoRawFd to transfer ownership to raw i32 file descriptors, allowing them to be used throughout the function without auto-closing.

[Copilot]

Pull request overview

This PR updates the uufuzz harness to avoid unsafe libc::dup by switching to the safe rustix::io::dup wrapper, adjusting fd handling accordingly.

Changes:

• Replace libc::dup calls with rustix::io::dup in fuzz/uufuzz/src/lib.rs and adjust error handling/ownership transfer.
• Add rustix as a dependency for uufuzz (and update fuzz/Cargo.lock).
• Add a fuzz/rust-toolchain.toml setting the fuzz workspace toolchain to nightly.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 5 comments.

File	Description
fuzz/uufuzz/src/lib.rs	Switch stdout/stderr/stdin duplication from libc::dup to rustix::io::dup and refactor fd handling.
fuzz/uufuzz/Cargo.toml	Add rustix dependency needed for rustix::io::dup.
fuzz/rust-toolchain.toml	Introduce a nightly toolchain pin for the fuzz workspace.
fuzz/Cargo.lock	Lockfile update to include rustix.

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[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?
Skipping an intermittent issue tests/tail/symlink (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)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)