elmPackages: minor cleanups

* Make sure generate script uses node2nix version from the nixpkgs
* regenerate code generated by node2nix

pkgs/development/compilers/elm/packages/generate-node-packages.sh

@@ -1,10 +1,12 @@
-#!/usr/bin/env nix-shell
-#! nix-shell -i bash -p nodePackages.node2nix
+#!/usr/bin/env bash
+
+ROOT="$(realpath "$(dirname -- "$(readlink -f -- "${BASH_SOURCE[0]}")")"/../../../../..)"
 
 set -eu -o pipefail
 
 rm -f node-env.nix
-node2nix --nodejs-10 \
+$(nix-build $ROOT -A nodePackages.node2nix --no-out-link)/bin/node2nix \
+    --nodejs-10 \
     -i node-packages.json \
     -o node-packages.nix \
     -c node-composition.nix \