fix: Set HelmBinary in execer constructor (#928)

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.

main.go

@@ -50,6 +50,7 @@ func main() {
 		cli.StringFlag{
 			Name:  "helm-binary, b",
 			Usage: "path to helm binary",
+			Value: "helm",
 		},
 		cli.StringFlag{
 			Name:  "file, f",