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/os-specific/linux/tp_smapi/default.nix

@@ -34,7 +34,7 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   passthru.updateScript = import ./update.nix {
-    inherit lib writeScript coreutils gnugrep jq curl common-updater-scripts;
+    inherit stdenv lib writeScript coreutils gnugrep jq curl common-updater-scripts;
   };
 
   meta = {