Fix NixOS detection
Fix NixOS detection

Use `/etc/os-release` instead of `/etc/NIXOS`.
The latter one does not exist on NixOS when using tmpfs as root.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust