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.

nixos/modules/virtualisation/amazon-init.nix

@@ -8,7 +8,7 @@ let
 
     export HOME=/root
     export PATH=${pkgs.lib.makeBinPath [ config.nix.package pkgs.systemd pkgs.gnugrep pkgs.gnused config.system.build.nixos-rebuild]}:$PATH
-    export NIX_PATH=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
+    export NIX_PATH=nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
 
     userData=/etc/ec2-metadata/user-data
 
@@ -18,9 +18,9 @@ let
       # that as the channel.
       if sed '/^\(#\|SSH_HOST_.*\)/d' < "$userData" | grep -q '\S'; then
         channels="$(grep '^###' "$userData" | sed 's|###\s*||')"
-        printf "%s" "$channels" | while read channel; do
+        while IFS= read -r channel; do
           echo "writing channel: $channel"
-        done
+        done < <(printf "%s\n" "$channels")
 
         if [[ -n "$channels" ]]; then
           printf "%s" "$channels" > /root/.nix-channels