julia.withPackages: fix some artifact builds

pkgs/development/julia-modules/python/extract_artifacts.py

···
           url = "{url}";
           sha256 = "{sha256}";
         }};
+        preUnpack = ''
+          mkdir unpacked
+          cd unpacked
+        '';
         sourceRoot = ".";
         dontConfigure = true;
         dontBuild = true;