This is a new argument to the Haskell builder's 'shellFor' which allows
the caller to adjust the 'genericBuilderArgs' after they are built.

One immediate application for this is that it allows users to disable
tests, enable benchmarks, etc. for _all_ packages in a given Haskell
package set.

This is often useful when so many of the derivations in the
package set have changed that most of the cache has been invalidated,
and there is no need for the user to run tests on their dependencies,
but they still want these dependencies available in their 'nix-shell'
environment for some package-under-development.

…-modifier

Adds Haskell generic builder argument modifier