in trigger.yml

  call_az_terra:
    needs: parse_and_call
    uses: ./.github/workflows/az_terra.yml
    with:
      environment: ${{ needs.parse_and_call.outputs.environment }}
    secrets:
      AZURE_CREDENTIALS: ${{ secrets.AZURE_CREDENTIALS }}

az_terra.yml (reusable workflow)

on:
  workflow_call:
    inputs:
      environment:
        required: true
        type: string
    secrets:
      AZURE_CREDENTIALS:
        required: true

The solution was provided by Benjamin W.