fix(nix): fetch micropython submodule in nix

should fix CI

nix/firmware.nix

-{ pkgsCross, python3, ninja, meson, gcc }:
+{ fetchFromGitHub, pkgsCross, python3, ninja, meson, gcc }:
 
+let
+  micropython = fetchFromGitHub {
+    owner = "micropython";
+    repo = "micropython";
+    rev = "62f004ba424920a01e60c7a9a064b8ec9cd69c12";
+    sha256 = "1v2aisrsy9g22jjlgq6yf5k111bvr3j9lr38jb77cgmhnp8sxfzi";
+  };
+in
   pkgsCross.armhf-embedded.stdenv.mkDerivation {
     name = "card10";
     src = ./..;
     postPatch = ''
       substituteInPlace card10-cross.ini \
         --replace arm-none-eabi arm-none-eabihf
+      ln -s ${micropython} lib/micropython/micropython
     '';
     nativeBuildInputs = [ ninja meson python3 gcc ];
     mesonFlags = [