Set `HELMFILE_HELM3=1` and run `helmfile` like `HELMFILE_HELM3=1 helmfile ...`.
When `HELMFILE_HELM3` is set, `test`, `template`, `delete`, `destroy` behave differently so that it works with Helm 3.
Note that `helmfile diff` doesn't work as `helm-diff` called under the hood doesn't support Helm v3 yet.
Ref #668