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.

pkg/state/state_test.go

@@ -524,7 +524,7 @@ func TestHelmState_flagsForUpgrade(t *testing.T) {
 				HelmDefaults:      tt.defaults,
 				valsRuntime:       valsRuntime,
 			}
-			helm := helmexec.New(logger, "default", &helmexec.ShellRunner{
+			helm := helmexec.New("helm", logger, "default", &helmexec.ShellRunner{
 				Logger: logger,
 			})
 			args, err := state.flagsForUpgrade(helm, tt.release, 0)