nodePackages: fix broken update script

The computed path was wrong and references within the file where
wrong as well due to the temporary directory.

pkgs/development/node-packages/generate.sh

@@ -1,15 +1,11 @@
 #!/usr/bin/env nix-shell
-#! nix-shell shell-generate.nix -i bash
+#! nix-shell -p nodePackages.node2nix -i bash
 set -eu -o pipefail
 
-cd "$NODE_NIXPKGS_PATH/pkgs/development/node-packages"
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+
+cd ${DIR}
 rm -f ./node-env.nix
 for version in 10 12 13; do
-    tmpdir=$(mktemp -d)
+  node2nix --nodejs-$version -i node-packages-v$version.json -o node-packages-v$version.nix -c composition-v$version.nix
-    node2nix --nodejs-$version -i node-packages-v$version.json -o $tmpdir/node-packages-v$version.nix -c $tmpdir/composition-v$version.nix
-    if [ $? -eq 0 ]; then
-        mv $tmpdir/node-packages-v$version.nix .
-        mv $tmpdir/composition-v$version.nix .
-    fi
 done
-cd -

pkgs/development/node-packages/shell-generate.nix

@@ -1,8 +0,0 @@
-{ nixpkgs ? import ../../.. {} }:
-with nixpkgs;
-mkShell {
-  buildInputs = [
-    bash nodePackages.node2nix
-  ];
-  NODE_NIXPKGS_PATH = builtins.toString ../../../.;
-}