pkgs/build-support/cabal: set LANG="en_US.UTF-8" for the Haddock run

Haskell packages that contain non-ascii characters in their .cabal file
or somewhere else in their haddock documentation fail to compile under
nixpkgs and usually flagged with noHaddock = true. I wanted to do the
same for modularArithmentic, when I realized that we just have to set
the locale to some UTF-8 compatible locale in build-support/cabal to fix
this issue correctly.