"amazon-init.nix" is not included in the default AMIs because it unconditionally runs a nixos-rebuild. Also, the test has never worked (http://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.ec2-config).
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.