skopeo: fix evaluation, change license from asl2 to asl20

pkgs/development/tools/skopeo/default.nix

···
     description = "A command line utility for various operations on container images and image repositories";
     homepage = "https://github.com/projectatomic/skopeo";
     maintainers = with stdenv.lib.maintainers; [ vdemeester ];
-    license = stdenv.lib.licenses.asl2;
+    license = stdenv.lib.licenses.asl20;
   };
 }