Reduce the size of the Nixpkgs/NixOS jobsets

This cuts nixpkgs:trunk from 78K to 31K jobs by disabling builds of
{node,go,python,emacs,coq,r,ocaml,perl}Packages. Thus these are now
only built if they are dependencies of top-level packages (such as
end-user applications). I left haskellPackages because they take
typically longer to build than the others (which are mostly
interpreted languages), so disabling them would be more painful to
users.

This is a temporary measure until we have a binary cache based Hydra
running on faster hardware, necessitated by the fact that evaluations
now regularly time out after 6 hours.