protobuf: 3.7 -> 3.8
protobuf: 3.7 -> 3.8

This is needed by newer versions of `tensorflow` and other ML tools, so we
should start working on an upgrade of the default versoin in the distribution.

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