Always use `os-release` rather than `/lib` to detect `NixOS` (bootstrap) [Two users over on zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Bootstrapping.20on.20NixOS) bumped into issues where NixOS wasn't being properly detected. I believe this was caused by the presence of `/lib` on their machines. `/lib` is not standard on NixOS but can still be created by users or scripts. We are already checking `/etc/os-release`. The presence of `ID=nixos` in it's output should be trustworthy and we shouldn't then go on to also check for `/lib`.