`env-cmd` example should mimic default `dotenv` behavior

[rreckonerr]

Is your proposal related to a problem?

By default env-cmd WILL override the globally defined env vars, which differs from the default dotenv behavior and adds confusion.

Describe the solution you'd like

We should provide --no-override flag to env-cmd in the staging build example

Describe alternatives you've considered

An alternative would be a first-class support for environment-specific .env files by CRA, but that's a matter of future releases

Additional context

• https://github.com/toddbluhm/env-cmd#--no-override-option
• https://github.com/motdotla/dotenv#what-happens-to-environment-variables-that-were-already-set