[PR Previews] Amplify Console Build Failure

**App ARN**:
arn:aws:amplify:eu-west-1:742504421912:apps/d2tzquzvkco6a2
**Region**:
eu-west-1
**Issue**
The backend environment build fails for PR previews on applications with social sign-in.

The issue **only** applies on the preview environments.

### `dev` branch preview settings:

### Env variables are set up as suggested in the [documentation](https://docs.aws.amazon.com/amplify/latest/userguide/environment-variables.html#creating-a-new-backend-environment-with-authentication-parameters):

**Error message**
`Error: auth headless is missing the following inputParams googleClientId, facebookAppId`

**Notes**
This issue seems to have been solved for "regular" branches from what I can read [here](https://github.com/aws-amplify/amplify-console/issues/206).
Would a custom amplify build script solve this issue?

Start by reproducing the PR preview backend build with the documented authentication environment variables and compare its settings with a regular branch, using the reported error as the checkpoint. Review the linked environment-variable documentation and issue #206; done means preview environments build successfully for applications using Google or Facebook sign-in.

Written by the indexing model from the issue text.