update.py throws an error

Steps to reproduce

PYTHONPATH=$(pwd):$PYTHONPATH ./tests/cachekey/update.py

will raise a BstError from get_last_task_error ("Getting the last task error is only supported when running tests")

What is the expected correct behavior?

The element.expected files should be updated with the new cache keys.

Possible fixes

update.py could mock BST_TEST_SUITE to the os.environ dict

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