polkadot: upgrade cargoSha256 and add maintainer note
polkadot: upgrade cargoSha256 and add maintainer note

While the package is broken and does not build, we can still recompute the
cargoSha256. I attempted to upgrade to the latest release, but that doesn't
build either.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs