Can't use NextJS preview functionality with contentful (not build previews)

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- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-console/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-console/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-console/blob/master/CONTRIBUTING.md).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.

### App Id

d3n8ctckktb5sn

### Region

us-east-1

### Amplify Console feature

Backend builds, Previews, Redirects

### Describe the bug

I've been trying to set up content previews in Next using [this guide](https://github.com/vercel/next.js/tree/canary/examples/cms-contentful). I just solved the issue of environment variables not being forwarded to the lambda@edge functions, but now when I use the preview functionality to generate a page the page itself returns a 404. This works when I build and run locally.

### Expected behavior

I expect to get the page that should be generated.

### Reproduction steps

Follow [this guide](https://github.com/vercel/next.js/tree/canary/examples/cms-contentful), specifically the part about creating `api/preview.js` and `api/exit-preview.js`

Follow [this comment](https://github.com/aws-amplify/amplify-console/issues/1987#issuecomment-885217940) to set up env var redirection

### Build Settings

default build settings

### Additional information

There aren't any guides on how to use this functionality with amplify, so I'm not even sure if it's doable.

Start with the Next.js cms-contentful example, especially api/preview.js and api/exit-preview.js, then review the environment-variable redirection setup linked in issue 1987. Reproduce the 404 using Amplify's default build settings and compare it with a local build; done means the preview-generated page loads successfully on Amplify.

Written by the indexing model from the issue text.