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/update.nix

@@ -1,6 +1,7 @@
-{ lib, writeScript, coreutils, curl, gnugrep, jq, common-updater-scripts }:
+{ stdenv, lib, writeScript, coreutils, curl, gnugrep, jq, common-updater-scripts }:
 
 writeScript "update-tp_smapi" ''
+#!${stdenv.shell}
 PATH=${lib.makeBinPath [ common-updater-scripts coreutils curl gnugrep jq ]}
 
 tags=`curl -s https://api.github.com/repos/evgeni/tp_smapi/tags`