Ensure proper environement variables isolation (AWS SDK v2 import is affected)

[medikoo]

Currently, all we do, is that we ensure that the state of environment variables is reset between test file runs. Some tests have isolation ensured either by internally using overrideEnv util or if they're based on runServerless.

Still, it appears that in AWS SDK v2 case, the existence of AWS_SDK_LOAD_CONFIG env variable may force tests to fail, as then AWS SDK on imports unconditionally assumes the existence of .aws folder, and as the home directory is redirected to an empty temp directory, it ends with failure (see serverless/serverless#11636 (comment)).

Ideally such errors should not surface