haskell-packages.nix: fix the implementation of 'cabalNoTest'

The previous implementation used the following tying-the-knot trickery to
override 'doCheck' to false for the given build:

  cabalNoTest = {
    mkDerivation = x: rec {
      final = self.cabal.mkDerivation (self: (x final) // { doCheck = false; });
    }.final;
  };

That seemed to work, but for some reason it caused trouble with some builds --
not all -- that use jailbreakCabal. The problem was the 'stdenv' attribute
couldn't be evaluated properly anymore:

  $ nix-build ~/pkgs/top-level/release-haskell.nix -A optparseApplicative.ghc6104.x86_64-linux --show-trace
  error: while evaluating the attribute `drvPath' at `/nix/store/qkj5cxknwspz8ak0ganm97zfr2bhksgn-nix-1.5.2pre3082_2398417/share/nix/corepkgs/derivation.nix:19:9':
  while evaluating the builtin function `derivationStrict':
  while instantiating the derivation named `haskell-optparse-applicative-ghc6.10.4-0.5.2.1' at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:40:13':
  while evaluating the derivation attribute `configurePhase' at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:107:13':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/lib/strings.nix:55:26':
  while evaluating the attribute `outPath' at `/nix/store/qkj5cxknwspz8ak0ganm97zfr2bhksgn-nix-1.5.2pre3082_2398417/share/nix/corepkgs/derivation.nix:18:9':
  while evaluating the builtin function `getAttr':
  while evaluating the builtin function `derivationStrict':
  while instantiating the derivation named `jailbreak-cabal-1.1' at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:40:13':
  while evaluating the derivation attribute `nativeBuildInputs' at `/home/simons/.nix-defexpr/pkgs/stdenv/generic/default.nix:76:17':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/lib/lists.nix:135:21':
  while evaluating the attribute `buildInputs' at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:22:17':
  while evaluating the builtin function `filter':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:22:60':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/top-level/haskell-packages.nix:119:17':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/lib/customisation.nix:61:22':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/lib/customisation.nix:56:24':
  while evaluating the builtin function `isAttrs':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/development/libraries/haskell/Cabal/1.14.0.nix:1:1':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/top-level/haskell-packages.nix:113:20':
  while evaluating the attribute `final' at `/home/simons/.nix-defexpr/pkgs/top-level/haskell-packages.nix:114:7':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/build-support/cabal/default.nix:9:5':
  while evaluating the function at `/home/simons/.nix-defexpr/pkgs/stdenv/generic/default.nix:51:24':
  while evaluating the attribute `meta.license' at `/home/simons/.nix-defexpr/pkgs/development/libraries/haskell/Cabal/1.14.0.nix:17:5':
  infinite recursion encountered

I tried to figure out why this happens, but eventually gave up. The new
implementation passes an argument called 'enableCheckPhase' to the Cabal
builder, which determines whether the user-specified doCheck value has any
effect or not. Now, a normal override can be used to disable unit testing.