Repository: create-react-app
Issue: REACT_APP_ environment variables not replaced for public folder js file during build time
Labels: needs triage,issue: bug report

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### Describe the bug

I wanna the react app to notify all the browse when there is a new version deployed so that the end user can try to refresh the opened page without any error message with lazy loading.

There is a loop to check a file `public/config.js` which include the latest app version in it:

```js
window.__env = {
    // other variables ....
    "CURRENT_VERSION": process.env.REACT_APP_VERSION
}
```

But it seems that the variable `process.env.REACT_APP_VERSION` is not replaced during the build time? 

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I followed the docs in https://create-react-app.dev/docs/adding-custom-environment-variables#referencing-environment-variables-in-the-html but it seems only work for the `public/index.html` file.

### Which terms did you search for in User Guide?

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I think these two are related issues.

https://github.com/facebook/create-react-app/issues/7953
https://github.com/facebook/create-react-app/issues/8004

And https://github.com/facebook/create-react-app/issues/8004#issuecomment-557268282 is mentioned that:

> This is fairly easily solved in most cases as a custom script within your project that is run before the react-scripts build.

But why the `public/index.html` works? I just think the custom script can be too tricky. A general mechanism is necessary.

### Environment

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```
Environment Info:

  current version of create-react-app: 4.0.2
  running from /Users/shanchuanxu/.config/yarn/global/node_modules/create-react-app

  System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
  Binaries:
    Node: 12.12.0 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.11.3 - /usr/local/bin/npm
  Browsers:
    Chrome: 88.0.4324.192
    Edge: Not Found
    Firefox: 84.0.2
    Safari: 14.0.3
  npmPackages:
    react: ^16.13.0 => 16.14.0
    react-dom: ^16.13.0 => 16.14.0
    react-scripts: 4.0.1 => 4.0.1
  npmGlobalPackages:
    create-react-app: Not Found
```

### Steps to reproduce

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### Expected behavior

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The env var in `public/config.js` can be replaced with a string.

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