nodePackages: Use repo tree as nixpkgs

This will make sure that generate.sh uses the correct version of
node2nix when building nodePackages, since the current version on 19.03
is 1.6.0 and the tree should be built with 1.7.0.

pkgs/development/node-packages/generate.sh

@@ -1,5 +1,6 @@
 #!/usr/bin/env nix-shell
-#! nix-shell -i bash -p nodePackages.node2nix
+#! nix-shell -I nixpkgs=../../.. -i bash -p nodePackages.node2nix
+# NOTE: Script must be run from the node-packages directory
 
 set -eu -o pipefail