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.

pkgs/development/libraries/grpc/default.nix

@@ -1,4 +1,5 @@
-{ lib, stdenv, fetchFromGitHub, fetchpatch, cmake, zlib, c-ares, pkg-config, re2, openssl, protobuf
+{ lib, stdenv, fetchFromGitHub, fetchpatch, buildPackages
+, cmake, zlib, c-ares, pkg-config, re2, openssl, protobuf, grpc
 , abseil-cpp, libnsl
 }:
 
@@ -20,7 +21,8 @@ stdenv.mkDerivation rec {
     })
   ];
 
-  nativeBuildInputs = [ cmake pkg-config ];
+  nativeBuildInputs = [ cmake pkg-config ]
+    ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) grpc;
   propagatedBuildInputs = [ c-ares re2 zlib abseil-cpp ];
   buildInputs = [ c-ares.cmake-config openssl protobuf ]
     ++ lib.optionals stdenv.isLinux [ libnsl ];
@@ -35,6 +37,8 @@ stdenv.mkDerivation rec {
       "-DBUILD_SHARED_LIBS=ON"
       "-DCMAKE_SKIP_BUILD_RPATH=OFF"
       "-DCMAKE_CXX_STANDARD=17"
+    ] ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
+      "-D_gRPC_PROTOBUF_PROTOC_EXECUTABLE=${buildPackages.protobuf}/bin/protoc"
     ];
 
   # CMake creates a build directory by default, this conflicts with the