Incorrect Hash in webpack production build when filenames uses [contenthash]

Hello,

We are looking to use the webpack plugin to wrap our SPA in a hashed inline script, and have it working locally, but when the webpack builds in production, the hash that is being generated is incorrect. I have narrowed down the issue to using a webpack config with a filename containing [contenthash]. This fails in a fresh, ejected create-react-app as well when building in production mode.

## Reproduction Repo:
https://github.com/todda00/test-csp

## Steps to Reproduce:

Clone reproduction repo
Navigate to repo root directory

Run the following:
```
npm install
npm run build
npx serve -s build
```

When the `output.filename` webpack config is changed to something static, the script is hashed correctly. Support for [contenthash] is common and required in many CDN served static production sites such as ours.