How to change env in 'backpack dev' or 'backpack build'

I want to change NODE_ENV in 'backpack build', this is my code, but it is not work.
```
cross-env NODE_ENV=test backpack build
```

Start by reproducing `cross-env NODE_ENV=test backpack build` and compare it with changing NODE_ENV for `backpack dev`. Determine how Backpack's build command receives environment variables. Done means documenting or correcting the command so the requested NODE_ENV is applied during the build.

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