gs: add environment credentials override fixture

[dtrifiro]

The cloud credentials messages in the logs are just debug messages, and are related to GCSFileSystem trying to get credentials from various sources (including environment) before giving up, see
https://github.com/fsspec/gcsfs/blob/fef6c0b098342a95990fd5aad8c3353d257f8d1b/gcsfs/credentials.py#L225-L242.

Note that these credentials are not used to access storage.googleapis.com because endpoint_url is overriden.

We could get rid of those warnings by setting GOOGLE_APPLICATION_CREDENTIALS to the path of a service account private key json, something similar to what we do with S3 with fake_creds_file. Trying to decide whether it's better to have a fixture for that here or in pytest-servers, although I'm opting for the former.

Originally posted by @dtrifiro in treeverse/dvc-gs#8 (comment)