update.nix: Run update scripts in parallel

To make updating large attribute sets faster, the update scripts
are now run in parallel.

Please note the following changes in semantics:

- The string passed to updateScript needs to be a path to an executable file.
- The updateScript can also be a list: the tail elements will then be passed
  to the head as command line arguments.

pkgs/development/web/nodejs/update.nix

@@ -1,4 +1,5 @@
-{ lib
+{ stdenv
+, lib
 , writeScript
 , coreutils
 , curl
@@ -11,6 +12,7 @@
 }:
 
 writeScript "update-nodejs" ''
+  #!${stdenv.shell}
   PATH=${lib.makeBinPath [ common-updater-scripts coreutils curl gnugrep jq gnupg nix ]}
 
   HOME=`mktemp -d`