grpc: fix cross
grpc: fix cross

I couldn't find any alternative to setting _gRPC_PROTOBUF_PROTOC_EXECUTABLE.
protobuf.cmake uses find_program when cross-compiling, which finds the
host platform's protoc instead of the build platform's.  I even tried
giving protobuf multiple outputs and not including the one with the
binary in buildInputs, but it didn't help.

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