amazon-init.nix: Don't run nixos-rebuild if we don't have to

nixos/modules/virtualisation/amazon-init.nix

@@ -6,7 +6,7 @@ let
   bootScript = pkgs.writeScript "bootscript.sh" ''
     #!${pkgs.stdenv.shell} -eux
 
-    echo "attempting to fetch configuration from user-data..."
+    echo "attempting to fetch configuration from EC2 user data..."
 
     export PATH=${config.nix.package}/bin:${pkgs.wget}/bin:${pkgs.systemd}/bin:${pkgs.gnugrep}/bin:${pkgs.gnused}/bin:${config.system.build.nixos-rebuild}/bin:$PATH
     export NIX_PATH=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
@@ -15,7 +15,7 @@ let
     wget -q --wait=1 --tries=0 --retry-connrefused -O - http://169.254.169.254/2011-01-01/user-data > "$userData"
 
     if [[ $? -eq 0 ]]; then
-      echo "user-data fetched"
+
       # If the user-data looks like it could be a nix expression,
       # copy it over. Also, look for a magic three-hash comment and set
       # that as the channel.
@@ -30,17 +30,17 @@ let
           nix-channel --update
         fi
 
-        echo "setting configuration"
+        echo "setting configuration from EC2 user data"
         cp "$userData" /etc/nixos/configuration.nix
       else
-        echo "user-data does not appear to be a nix expression; ignoring"
+        echo "user data does not appear to be a Nix expression; ignoring"
+        exit
       fi
     else
-      echo "failed to fetch user-data"
+      echo "failed to fetch user data"
+      exit
     fi
 
-    type -f nixos-rebuild
-
     nixos-rebuild switch
   '';
 in {