feat: add process.env.TRIGGER_ENV

[youminkim]

Is your feature request related to a problem? Please describe.

We frequently check environment (e.g. production, staging) but there is no default environment variable from trigger.dev runtime.

Describe the solution you'd like to see

Add process.env.TRIGGER_ENV with production, staging and development.

example usage

export function isProd() {
  return (
    process.env.VERCEL_ENV === 'production' || 
    process.env.TRIGGER_ENV === 'production'
  );
}

Describe alternate solutions

Add MY_TRIGGER_ENV manually into project environment settings.

Additional information

https://discord.com/channels/1066956501299777596/1316791509818937354/1316791509818937354