ci: Pin to dedicated nodes

Now that there are several production workloads on the Raspberry Pi
cluster, we don't want intense jobs like this running on them.  To
ensure this job runs on a dedicated node, we need to use a label
expression that matches the appropriate nodes.

ci/podTemplate.yaml

@@ -13,6 +13,8 @@ spec:
       name: ssh-known-hosts
       subPath: ssh_known_hosts
   hostUsers: false
+  nodeSelector:
+    du5t1n.me/jenkins:
   volumes:
   - name: ssh-known-hosts
     configMap: