How do I use GitHub Actions envrionment variables in a job called with workflow_call

You should define that as input params in the workflow_call section and pass them in the caller workflow. Like:

on:
  workflow_call:
    inputs:
      AWS_REGION: { required: true, type: string }
      AWS_ACCESS_KEY_ID: { required: true, type: string }
      AWS_SECRET_ACCESS_KEY: { required: true, type: string }

and use it like:

    with:
      aws-access-key-id: ${{ inputs.AWS_ACCESS_KEY_ID }}
      aws-secret-access-key: ${{ inputs.AWS_SECRET_ACCESS_KEY }}
      aws-region: ${{ inputs.AWS_REGION }}

In this way you could lost the ability to call with a workflow_dispatch. In order to support that also, you could try this approach:

    with:
      aws-access-key-id: ${{ inputs.AWS_ACCESS_KEY_ID ||  secrets.AWS_ACCESS_KEY_ID }}
      aws-secret-access-key: ${{ inputs.AWS_SECRET_ACCESS_KEY ||  secrets.AWS_SECRET_ACCESS_KEY }}
      aws-region: ${{ inputs.AWS_REGION }}

Not tested, may require an intermediary step to resolve this part

The workflow_call needs secrets to be sent separately:

A map of the secrets that can be used in the called workflow.

Example

on:
  workflow_call:
    secrets:
      access-token:
        description: 'A token passed from the caller workflow'
        required: false

jobs:

  pass-secret-to-action:
    runs-on: ubuntu-latest
    steps:
    # passing the secret to an action
      - name: Pass the received secret to an action
        uses: ./.github/actions/my-action
        with:
          token: ${{ secrets.access-token }}

  # passing the secret to a nested reusable workflow
  pass-secret-to-workflow:
    uses: ./.github/workflows/my-workflow
    secrets:
       token: ${{ secrets.access-token }}

Reference

Moreover, according to this blog post, you can inherit secrets now:

GitHub Actions simplifies using secrets with reusable workflows with the secrets: inherit keyword.

Previously when passing secrets to a reusable workflow, you had to pass each secret as a separate argument. Now you can simply pass the secrets: inherit to the reusable workflow and the secrets will be inherited from the calling workflow.

to use aws-actions/configure-aws-credentials in a reusable workflow you explicitly pass the required credentials:

# reusable run_tests.yml

name: Run Tests
on:
  workflow_call: # allows calling this workflow from another workflow
    secrets:
      AWS_ACCESS_KEY_ID: { required: true }
      AWS_SECRET_ACCESS_KEY: { required: true }

jobs:
  tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Configure AWS Credentials
        uses: aws-actions/configure-aws-credentials@v4
        with:
          aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
          aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          aws-region: eu-west-1
      # - ... rest of the steps...

# your caller workflow (main workflow)

jobs:
  tests:
    uses: ./.github/workflows/run_tests.yml
    secrets:
      AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
      AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
  build-and-push:
  ...

took me few hours to figure this out...