std::thread::sleep needs target_os

Co-authored-by: Sylvestre Ledru <sledru@mozilla.com>

tests/by-util/test_install.rs

@@ -2,6 +2,7 @@ use crate::common::util::*;
 use filetime::FileTime;
 use rust_users::*;
 use std::os::unix::fs::PermissionsExt;
+#[cfg(target_os = "linux")]
 use std::thread::sleep;
 
 #[test]