haddock: Pass --no-tmp-comp-dir by default

With "Hi Haddock", we want to re-use GHC's compilation output (in
particular, the generated interface files) to generate documentation
from. However, haddock by default will use a temporary location to build
sources, and will therefore fail to find the interface files produced by
GHC.

To avoid recompilation, we need to instruct Cabal to pass
--no-tmp-comp-dir to Haddock so as to not use a temporary directory and
thus re-use existing interface files.