ci: Mount ssh_known_hosts in client build pod

Apparently, the although the tooltip says " Volumes get mounted in all
containers with the specified mount path," volumes defined in the
default Pod Template in the Jenkins Kubernetes Cloud configuration
actually only get mounted in the JNLP agent container.  Containers
defined in the pod template YAML document are not included in this
interpretation of "all."

ci/clientPodTemplate.yaml

@@ -6,4 +6,12 @@ spec:
     - cat
     stdin: true
     tty: true
+    volumeMounts:
+    - mountPath: /etc/ssh/ssh_known_hosts
+      name: ssh-known-hosts
+      subPath: ssh_known_hosts
   hostUsers: false
+  volumes:
+  - name: ssh-known-hosts
+    configMap:
+      name: ssh-known-hosts