Local environment variables interfere with tests

### Describe the issue

When running the test suite, if the `AWS_PROFILE` version is set to something locally reasonable, some tests fail.

I'd like to suggest going through the [list of supported variables](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html) and unsetting all of them as part of a global test setup step - to ensure more repeatable builds. It also seems possible that this problem may cause local test success for developers where tests should really fail, as well as the reverse, which is the symptom I've encountered.

### Additional Information/Context

This came up on the AUR - you can see the discussion [on the AWS CLI project there](https://aur.archlinux.org/packages/aws-cli-v2).

### CLI version used

2.15.40

### Environment details (OS name and version, etc.)

Arch GNU/Linux, current, installed using AUR package (kernel 6.8.7)