app.kubernetes.io/instance should be overridable using helm values

[rouke-broersma]

app.kubernetes.io/instance is supposed to be used to define a unique identifier for an instance of an application. This is hardcoded to 'default' in the chart. This means you cannot use the instance label to differentiate two instances of tekton in the same cluster. This also causes conflicts with argocd because argocd wants to fill this field with the Argo Application 'instance' that deploys tekton.