digimend: prepend kernel.makeFlags to makeFlags

pkgs/os-specific/linux/digimend/default.nix

···
     sha256 = "1l54j85540386a8aypqka7p5hy1b63cwmpsscv9rmmf10f78v8mm";
   };
 
-  INSTALL_MOD_PATH = "\${out}";
-
   postPatch = ''
     sed 's/udevadm /true /' -i Makefile
     sed 's/depmod /true /' -i Makefile
···
     rm -r $out/lib/udev
   '';
 
-  makeFlags = [
+  makeFlags = kernel.makeFlags ++ [
     "KVERSION=${kernel.modDirVersion}"
     "KDIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
     "DESTDIR=${placeholder "out"}"
+    "INSTALL_MOD_PATH=${placeholder "out"}"
   ];
 
   meta = with lib; {