feat(3p/cpp/googleapis): Add Nix build instructions

third_party/cpp/googleapis/default.nix

@@ -0,0 +1,24 @@
+# This library contains generated gRPC implementations for Google's
+# public libraries.
+{ pkgs, ... }:
+
+let
+  inherit (pkgs) fetchFromGitHub;
+  stdenv = with pkgs; overrideCC pkgs.stdenv clang_9;
+in stdenv.mkDerivation {
+  name = "googleapis-cpp";
+  src = ./.;
+
+  GOOGLEAPIS_DIR = fetchFromGitHub {
+    owner = "googleapis";
+    repo = "googleapis";
+    rev = "0aba1900ffef672ec5f0da677cf590ee5686e13b";
+    sha256 = "1174mvipmzap4h8as1cl44y1kq7ikipdicnmnswv5yswgkwla84c";
+  };
+
+  buildInputs = with pkgs; [
+    c-ares c-ares.cmake-config grpc protobuf openssl zlib
+  ];
+
+  nativeBuildInputs = with pkgs; [ cmake pkgconfig ];
+}