post render plugin doesn't seem to work

I am trying to follow the post-render example using kustomize. I am pointing directly to this online repo:

```
apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
name: kustomized-helm-ex
namespace: argocd
spec:
destination:
namespace: default
server: https://kubernetes.default.svc
project: default
source:
repoURL: https://github.com/argoproj/argocd-example-apps
targetRevision: master
path: plugins/kustomized-helm
plugin:
name: kustomized-helm
syncPolicy:
automated:
prune: true
selfHeal: true
syncOptions:
- CreateNamespace=true
```

I get:

```
status:
conditions:
- lastTransitionTime: "2021-08-31T15:30:07Z"
message: 'rpc error: code = Unknown desc = Manifest generation error (cached):
`sh -c helm template --release-name $ARGOCD_APP_NAME --namespace $ARGOCD_APP_NAMESPACE
--include-crds . > all.yaml && kustomize build` failed exit status 1: Error:
no matches for OriginalId apps_v1_Deployment|~X|release-name-helm-guestbook;
no matches for CurrentId apps_v1_Deployment|~X|release-name-helm-guestbook;
failed to find unique target for patch apps_v1_Deployment|release-name-helm-guestbook'
```

I did some research, and found that someone else who got this says it means the patch is targeting something that doesn't exist (in this case, release-name-helmguestbook deployment). But why would that be? we don't need to separately deploy the helm chart right? we cannot do both helm and plugin in the same app object.

unless the example readme is leaving out a step i would think you can use it "as is".

I think the plugin installed successfully, since it is indeed trying to run the provided generate command (helm template --args)

Start with the plugins/kustomized-helm example repository path and the provided Application manifest, then reproduce the reported helm template and kustomize build command. The work is done when the example can generate manifests successfully from the Application as documented, without the reported Deployment patch error.

Written by the indexing model from the issue text.