fix(nix): remove submodule dir first

nix/firmware.nix

@@ -6,6 +6,7 @@ let
     repo = "micropython";
     rev = "62f004ba424920a01e60c7a9a064b8ec9cd69c12";
     sha256 = "1v2aisrsy9g22jjlgq6yf5k111bvr3j9lr38jb77cgmhnp8sxfzi";
+    fetchSubmodules = true;
   };
 in
   pkgsCross.armhf-embedded.stdenv.mkDerivation {
@@ -14,7 +15,8 @@ in
     postPatch = ''
       substituteInPlace card10-cross.ini \
         --replace arm-none-eabi arm-none-eabihf
-      ln -s ${micropython} lib/micropython/micropython
+      rmdir lib/micropython/micropython
+      ln -s ${micropython}/ lib/micropython/micropython
     '';
     nativeBuildInputs = [ ninja meson python3 gcc ];
     mesonFlags = [