Provide Windows and macOS binaries of cabal-docspec

[cydparser]

if someone asked nicely, I could provide windows and osx binaries too (haskell/cabal#2327 (comment))

Pretty please :)

[phadej]

Most likely won't happen. I want to be able to build binaries without relying on 3rd party services (e.g. using local docker alpine container for linux builds). Perfectly in controller environment (like docker container). Windows is already tricky, but macOS is out of reach.