How to enable auto population of ENV variables while deploying from CI/CD

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting questions](https://github.com/aws-amplify/amplify-hosting/blob/master/CONTRIBUTING.md).
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

d33cv943mt6efp

### AWS Region

us-east-1

### Amplify Hosting feature

Backend builds

### Question

I have multiple functions in my Amplify Project. I need to add some new ENV variables.

I have a production environment mapped to a production branch.
I make changes to my code, add a new variable, merge the code to production branch in git and do a git push.

Now the CI/CD pipeline setup in Amplify for the production branch triggers a backend build and push.
When a new ENV variable is added to a Backend function, amplify asks for the env variable when doing amplify push. How does amplify hosting access the ENV variable when building through CI/CD pipeline. I do not want to manually push to my prod environment.

Start with FAQ.md and the Amplify Hosting Backend builds flow described in the issue, then trace how a production branch CI/CD deployment handles newly added function environment variables. Done means documenting the supported way to provide those variables during deployment without requiring a manual production push.

Written by the indexing model from the issue text.