How to set up correct Pusher Key for proper environment?

in bootstrap.js

window.Echo = new Echo({
    broadcaster: "pusher",
    key: process.env.MIX_PUSHER_APP_KEY,
    cluster: process.env.MIX_PUSHER_APP_CLUSTER,
    encrypted: true
});

I have pusher-staging and pusher-production app key and app cluster. How do I make sure that when deploying to vapor correct pusher key is used?