amazon-init.nix: Use makeBinPath

This also fixes the incorrect use of 'dev' outputs from
config.nix.package and pkgs.systemd.

nixos/modules/virtualisation/amazon-init.nix

···
 
     echo "attempting to fetch configuration from EC2 user data..."
 
-    export PATH=${config.nix.package}/bin:${pkgs.systemd}/bin:${pkgs.gnugrep}/bin:${pkgs.gnused}/bin:${config.system.build.nixos-rebuild}/bin:$PATH
+    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
 
     userData=/etc/ec2-metadata/user-data