uucore: fix correct mkdirat implementation to use nix crate's mkdirat function

[mattsu2020]

The commit fixes an issue where the mkdir_at method was incorrectly using the raw libc mkdirat function instead of the nix crate's mkdirat function. This change ensures proper error handling and type safety by using the nix crate's wrapper, which provides better integration with Rust's type system and error handling patterns.

[sylvestre]

what was the impact for users?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes 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/follow-name (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[mattsu2020]

what was the impact for users?

No end-user behavioral change is intended.
This is an internal refactor replacing direct libc calls with nix wrappers.
CLI behavior, output, and exit codes remain the same.
The main impact is reduced unsafe surface and improved maintainability.

[sylvestre]

mkdir_at method was incorrectly using the raw libc mkdirat function instead of the nix crate's mkdirat function

I was confused by the "incorrectly" :)