this can be added with the environement variable: AMPLIFY_MONOREPO_APP_ROOT
https://docs.aws.amazon.com/amplify/latest/userguide/environment-variables.html
AMPLIFY_MONOREPO_APP_ROOT
The path to use to specify the app root of a monorepo app, relative to the root of your repository.
apps/react-app
The AMPLIFY_MONOREPO_APP_ROOT environment variable must have the same value as the appRoot key inside the build spec. –