hackage2nix: set meta.hydraPlatforms for extra-packages to "none" by default
hackage2nix: set meta.hydraPlatforms for extra-packages to "none" by default

Those packages are in the packgae set only to be used during overrides, i.e. to
make stuff compile with older versions of GHC. We don't want to build them by
default, because oftentimes these older versions don't work with the latest
version.

Fuuzetsu/cabal2nix,bennofs/cabal2nix