mtdutils: fix cross compilation by setting AR in makeFlags

pkgs/tools/filesystems/mtdutils/default.nix

···
     (enableFeature doCheck "tests")
   ];
 
+  makeFlags = [
+    "AR:=$(AR)"
+  ];
+
   doCheck = stdenv.hostPlatform == stdenv.buildPlatform;
 
   outputs = [ "out" "dev" ];