Prevent wheel builds using the existing mirror

I think we have a situation where a wheel build has somehow put a
corrupt wheel into our mirrors, but the next run seems to grab the
corrupt wheel from the mirror again ...

Remove the wheel mirror from the pip configuration before starting the
build so we're actually building directly from upstream, which is what
is intended.

Change-Id: Icb590e805d0d29130bff99335590d1d0f646e4e4

roles/build-wheels/tasks/main.yaml

@@ -6,5 +6,11 @@
     state: directory
   become: yes
 
+- name: Prevent using existing wheel mirror
+  lineinfile:
+    path: /etc/pip.conf
+    state: absent
+    regexp: '^extra-index-url.*$'
+
 - name: Build the wheel mirror
   script: wheel-build.sh {{ wheel_dir }} {{ wheel_python }}