jenkins: Add Argo CD pre-sync hook

Argo CD will delete and re-create this Job each time it synchronizes the
*jenkins* application.  The job creates a snapshot of the Jenkins volume
using an HTTP request to the Longhorn UI.

jenkins/kustomization.yaml

@@ -7,6 +7,7 @@ labels:
 
 resources:
 - jenkins.yaml
+- argocd-sync-hook.yaml
 
 configMapGenerator:
 - name: ssh-known-hosts