Fix side effect of setting env variables 'AWS_ACCESS_KEY_ID' and 'AWS_SECRET_ACCESS_KEY'

[thomasmarwitz]

When using the S3FSStore, the environment variables 'AWS_ACCESS_KEY_ID' and 'AWS_SECRET_ACCESS_KEY' are currently set automatically based on the values provided in the url to create the store.

This is a side effect since the credentials are passed explicitly.

Especially in CI, this can be very annoying. Consider this workflow:

1. AWS credentials are assumed via OpenID Connect, the GH action saves the credentials to the default AWS_* env variables
2. A test runs that assumes a role with the given credentials.
3. Inside this test, a minimalkv bucket S3FSStore is created and overwrites the AWS_* env variables
4. Another test now finds the credentials of the assumed role rather than the original ones that were obtained through OpenID Connect