Cabal: Don't try to use the user's ~/.cabal

When there's no ~/.cabal or ~/.cabal/config, cabal tries to create it.

We don't want (or need) that for our `ghc -M` based build;
an empty (default) cabal config does fine.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>