As the only consequence of isSystemUser is that if the uid is null then it's allocated below 500, if a user has uid = something below 500 then we don't require isSystemUser to be set. Motivation: https://github.com/NixOS/nixpkgs/issues/112647
The library does not depend on stdenv, that `stdenv` exposes `lib` is an artifact of the ancient origins of nixpkgs.
On slow hardware, the test was flaky. This improves it.
