We have having constant issues with the bionic arm64 mirror shutting
itself off. Before we go too far down the path of debugging what
appears to be a kernel oops issue, let's rebuild it as focal.
Update the sources list in the base. Update the testing to use a
focal node.
Change-Id: I5b7106e2263010ff353e8a1de43e73b0c0ec57e1
4ea57feabb