jenkins: Pin to x86_64 nodes

The Raspberry Pi usually has the most free RAM of all the Kubernetes
nodes, so pods tend to get assigned there even when it would not be
appropriate.  Jenkins, for example definitely does not need to run
there, so let's force it to run on the bigger nodes.

jenkins/jenkins.yaml

@@ -137,6 +137,8 @@ spec:
         volumeMounts:
         - name: jenkins-data
           mountPath: /var/jenkins_home
+      nodeSelector:
+        kubernetes.io/arch: amd64
       securityContext:
         runAsUser: 1000
         fsGroup: 1000