fix: clear session token env var if present for non-session credentials

[clareliguori]

Related to #57, also document how to use different AWS accounts in the readme

This change is only necessary where the first action requires a session token, but the second action doesn't. For example:

    - name: Configure AWS credentials from Production account
      uses: aws-actions/configure-aws-credentials@v1
      with:
        aws-access-key-id: ${{ secrets.TEST_AWS_ACCESS_KEY_ID }}
        aws-secret-access-key: ${{ secrets.TEST_AWS_SECRET_ACCESS_KEY }}
        role-to-assume: ${{ secrets.AWS_ROLE_TO_ASSUME }}
        aws-region: us-west-2

    - name: Configure AWS credentials from Production account
      uses: aws-actions/configure-aws-credentials@v1
      with:
        aws-access-key-id: ${{ secrets.PROD_AWS_ACCESS_KEY_ID }}
        aws-secret-access-key: ${{ secrets.PROD_AWS_SECRET_ACCESS_KEY }}
        aws-region: us-west-2

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[piradeepk]

Ship it! 💯