This overrides the default helm command, if provided, as soon as possible.
This way it is already used in `visitStates`.
I ran into an issue using `HELM3` and `--helm-binary` together with helm-secrets.
I previously used tillerless, which i could now remove. This however caused `DecryptSecret` to fail, as it would still use the helm2 binary; because it runs before the first `helm.SetHelmBinary` call.
While helm-secrets is not fully helm3 compatible yet, its uses within helmfile are, and i was able to just install it as a helm3 plugin.
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