Add KUBECONFIG environment variable for plugin invocation

This also allows an existing variable to be overridden by the
--kubeconfig flag.

Signed-off-by: Janis Meybohm <jayme-github@users.noreply.github.com>

pkg/plugin/plugin.go

@@ -197,4 +197,8 @@ func SetupPluginEnv(settings helm_env.EnvSettings,
 	if settings.Debug {
 		os.Setenv("HELM_DEBUG", "1")
 	}
+
+	if settings.KubeConfig != "" {
+		os.Setenv("KUBECONFIG", settings.KubeConfig)
+	}
 }