capnproto: fix cross
capnproto: fix cross

capnproto tries to compile some capnproto files, so when
cross-compiling we need to give it the build platform's capnproto
binary or it'll try to execute the one for the host platform.

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