arti: running_as_setuid: fix MacOs build

libc::getuid and geteuid are marked unsafe, even though I think they could be safe. So the previous code didn't build.

This time I have at least checked that the thing compiles on Linux, by locally adjusting the #[cfg].

This failed in https://gitlab.torproject.org/tpo/core/arti/-/jobs/169701. I don't understand why this job isn't running, and therefore failing, for every run on main.