virtualization/amazon-init: fix logging, nix path

The missing `\n` in the printf format string prevented multiple channels from
being logged.

The missing `nixpkgs=` in the `NIX_PATH` prevented `nixos-rebuild` from working
if the system configuration has any reference to `nixpkgs`.

Additionally:

* Use process substitution instead of piping printf to avoid creating a subshell.
* Set an empty `IFS` to avoid word splitting.
* Add the `-r` flag to `read` to avoid mangling backslashes.