minixml: fix darwin build

The default configuration tries to build an intel+arm version on
darwin-aarch64 which fails, but which we don't need.
Remove the explicit architecture flags to fix the build.

pkgs/development/libraries/minixml/default.nix

···
     sha256 = "sha256-l7GUA+vlSECi/72eU3Y9COpGtLTRh3vYcHUi+uRkCn8=";
   };
 
+  # remove the -arch flags which are set by default in the build
+  configureFlags = lib.optionals stdenv.isDarwin [
+    "--with-archflags=\"-mmacosx-version-min=10.14\""
+  ];
+
   enableParallelBuilding = true;
 
   meta = with lib; {