Switch cirrus ubuntu image to ubuntu:latest

Done to fix a problem where clang fails to install due to an
inconsistency in the apt sources used by the ubuntu kinetic (a.k.a
devel) Docker image.  This means an update to clang via the
kinetic-proposed source makes it uninstallable on that image
until the proposed change makes it to the kinetic one.

Switching to ubuntu:latest means we won't be quite as leading-edge
but it's less likely to break unexpectedly.

.cirrus.yml

@@ -76,7 +76,7 @@ gcc_task:
 ubuntu_task:
   name: ubuntu-clang
   container:
-    image: ubuntu:devel
+    image: ubuntu:latest
     cpu: 2
     memory: 1G