Correct artifact return url for python/js

The recent change to return the artifact URLs for python/js builds
omitted the artifacts/ directory from the URL.

Change-Id: If86749533e73dbfd17317acba22dbd05dff34abb

playbooks/python/tarball-post.yaml

@@ -46,6 +46,6 @@
           zuul:
             artifacts:
               - name: "python_wheel"
-                url: "{{ item.path | basename }}"
+                url: "artifacts/{{ item.path | basename }}"
                 metadata:
                   type: python_wheel

roles/fetch-javascript-content-tarball/tasks/main.yaml

@@ -51,6 +51,6 @@
       zuul:
         artifacts:
           - name: javascript_content
-            url: "{{ item.path | basename }}"
+            url: "artifacts/{{ item.path | basename }}"
             metadata:
               type: javascript_content