haskell-generic-builder: use "./Setup install" to install packages that export no library

In theory, "./Setup copy" should do the same thing, but experience has shown
that library authors have no clue about the distinction between the 'copy' and
the 'install' stage, and usually don't bother making 'copy' work the way it's
supposed to (see git-annex for a concreet example). Using 'install' whenever we
can avoid this kind of trouble, hopefully.