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/tools/build-managers/gup/default.nix

@@ -16,6 +16,7 @@ stdenv.mkDerivation rec {
     cp -r python/bin $out/bin
   '';
   passthru.updateScript = ''
+    #!${stdenv.shell}
     set -e
     echo
     cd ${toString ./.}