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/applications/networking/browsers/firefox/update.nix

@@ -1,4 +1,5 @@
 { writeScript
+, stdenv
 , lib
 , xidel
 , common-updater-scripts
@@ -13,6 +14,7 @@
 }:
 
 writeScript "update-${attrPath}" ''
+  #!${stdenv.shell}
   PATH=${lib.makeBinPath [ common-updater-scripts coreutils curl gnugrep gnused xidel ]}
 
   url=${baseUrl}