PYTHON-6051 Convert drivers-evergreen-tools to a git submodule and pin the GitHub Actions reference

[blink1073]

PYTHON-6051

Changes in this PR

Enforces a pinned version for all usages of drivers-evergreen-tools, and that it is always available in a known location using git submodules. Dependabot manages all of the version pins.

• Evergreen scripts initialize the submodule in place instead of re-cloning into a sibling directory, and no longer delete it; DRIVERS_TOOLS defaults to the in-repo checkout (env override still wins).
• just install initializes the submodule for local development; CONTRIBUTING.md drops the manual clone instructions.
• GitHub Actions check out with submodules: true and pin all uses: refs to the release SHA.

Test Plan

Standard tests

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? — Not needed; no driver code changes.
• Is there test coverage? — N/A (CI/build tooling).
• Is any followup work tracked in a JIRA ticket? If so, add link(s). — No followup required; dependabot maintains the pin.

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?