download-artifacts: correct build selection

The most recent build is the first entry in the result set, not
the last -- the set is returned in descending order.

Change-Id: I5fb788e5cf68a194e90e23196d3eeef6169c9c59

roles/download-artifact/tasks/main.yaml

@@ -4,7 +4,7 @@
   register: build
 - name: Parse build response
   set_fact:
-    build: "{{ build.json[-1] }}"
+    build: "{{ build.json[0] }}"
 - name: Download archive
   uri:
     url: "{{ artifact.url }}"