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.
This way we have the benefit of the usual Nixpkgs style, and gain a
slight reduction in amount of code in the updater.
Also use callPackage to reduce duplication of the dependencies of the
update script and use makeBinPath to make things neater.