ESC: Improve docs on `esc env run` usage

The [esc/environments/dynamic-environment-variables/ page](https://www.pulumi.com/docs/esc/environments/dynamic-environment-variables/) could be more comprehensive.  See Torian's [Getting Started](https://www.pulumi.com/docs/esc/get-started/esc-run-command/) and [Blog](https://www.pulumi.com/blog/esc-env-run-aws/).