fix: a couple of bugs in get_session_credentials

[paulfouquet]

File "/app/scripts/aws/aws_helper.py", line 103, in get_session_credentials
    credentials: AwsFrozenCredentials = get_session(prefix).get_frozen_credentials()
AttributeError: 'Session' object has no attribute 'get_frozen_credentials'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/app/scripts/non_visual_qa.py", line 161, in <module>
    main()
  File "/app/scripts/non_visual_qa.py", line 157, in main
    non_visual_qa(source)
  File "/app/scripts/non_visual_qa.py", line 145, in non_visual_qa
    file_check.run()
  File "/app/scripts/non_visual_qa.py", line 99, in run
    gdalinfo_process = run_gdal(gdalinfo_command, self.path)
  File "/app/scripts/gdal/gdal_helper.py", line 63, in run_gdal
    credentials = get_session_credentials(input_file)
  File "/app/scripts/aws/aws_helper.py", line 105, in get_session_credentials
    except client_sts.meta.client.exceptions.InvalidIdentityTokenException as e:
AttributeError: 'ClientMeta' object has no attribute 'client'