Haddock test fails on nix

[myroslambda]

The haddock tests uses cabal exec and fails on nix.

The solution implemented should also work with our current setup and in the CI server.

[myroslambda]

Related: #95

[zoranbosnjak]

@myroslambda I can try to fix this.

Does this change breaks the CI server build?

-  (code, out, _err) <- readProcessWithExitCode "cabal" cabalArgs ""
+  (code, out, _err) <- readProcessWithExitCode "haddock" files ""

If this change is OK, then it should not be too difficult to replace it over the code base. Assuming that this is the only nix build problem.

If on the other hand this change breaks the "CI server build", then I am not sure if I can help with this problem.

Anyway, this is the procedure, to reproduce the current nix-build problem.

# for each subproject, for example:
cd keera-hails-reactivevalues
cabal2nix --shell . > default.nix
nix-build