This forces all haskell-stack build-style using templates to use
the system ghc and never download any binary distributions even
if the version does not match. This is usually fine as long as
the stackage used for the template is recent enough. If it's
not, it should probably be bumped anyway.
This also enables stack to work on all platforms, even those for
which stack does not offer any binary ghc downloads, as long as
the system ghc is provided, e.g. for ppc64le.
