fix: mock away TASKCLUSTER_{CLIENT_ID,ACCESS_TOKEN}

[bhearsum]

I happened to run tests in a termimal where I was signed into a cluster, and got a test failure because of it:

E       AssertionError: expected call not found.
E       Expected: mock({'rootUrl': 'http://taskcluster-prod'})
E         Actual: mock({'rootUrl': 'http://taskcluster-prod', 'credentials': ...})