PYTHON-6083 [v4.18] Latest server binary downloads require private S3 - #3080

PYTHON-6083

Backport of #3076 to v4.18 (upstream commit 916f320).

Changes in this PR

Ensure "latest" server binary downloads come from a private S3 bucket by providing AWS credentials (DRIVERS-3628). Evergreen tasks downloading VERSION=latest now assume drivers-test-secrets-role before the download and pass the AWS credentials to the orchestration step.

CI-only change: .evergreen/ configuration and CONTRIBUTING.md — no driver code is touched.

Test Plan

• Regenerated the Evergreen configs and verified assume ec2 role is inserted for every latest task and nowhere else.
• Patch build with one latest task confirms the S3 download succeeds with no FALLBACK: banner.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? — N/A, infrastructure
• Is there test coverage? — N/A, standard tests
• Is any followup work tracked in a JIRA ticket? — N/A

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?