jenkins: Set instance label for Argo CD

Argo CD wants every resource managed by an application to have that
application's name as the value of the `app.kubernetes.io/instance`
label.

jenkins/kustomization.yaml

@@ -1,6 +1,10 @@
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 
+labels:
+- pairs:
+    app.kubernetes.io/instance: jenkins
+
 resources:
 - jenkins.yaml