Initial Environment variables documentation

[truongnhatanh7]

The documentation about environment variables misses these 2 things:

• No documentation for loading from .env file to trigger.dev cloud environment at initial load
• When using envvars.retrieve, there's no direct way of knowing current environment. I could workaround that by create another var, but that introduces another var so it's not that good.

Path: /deploy-environment-variables