test(NODE-7574): use mapping refspec when fetching tag in switch source

[tadjik1]

Description

Summary of Changes

The switch source Evergreen function was broken for tag-based SOURCE_REV values (e.g. refs/tags/v7.0.0), causing all four test-bson-latest-driver-7.0.0-* tasks to fail:

git fetch origin refs/tags/v7.0.0
* tag  v7.0.0 -> FETCH_HEAD
git checkout refs/tags/v7.0.0
error: pathspec 'refs/tags/v7.0.0' did not match any file(s) known to git

Root cause: git fetch origin <refspec> without a destination stores the result in FETCH_HEAD only - it does not create a local ref at refs/tags/v7.0.0. The subsequent git checkout refs/tags/v7.0.0 then fails because that local ref doesn't exist.

This previously worked because the Evergreen initial clone included all remote tags, so refs/tags/v7.0.0 already existed locally before the fetch ran. A change in the runner/agent setup (shallower clone without --tags) removed that implicit dependency.

Double check the following

• Lint is passing (npm run check:lint)
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[Copilot]

Pull request overview

Fixes Evergreen’s switch source function so tag-based SOURCE_REV values (e.g., refs/tags/v7.0.0) can be checked out reliably in shallow clones by fetching the ref into a local ref (instead of leaving it only in FETCH_HEAD).

Changes:

• Update git fetch in switch source to use a mapping refspec (<src>:<dst>) so the fetched tag ref exists locally for git checkout.
• Apply the same change to both the source (config.in.yml) and generated (config.yml) Evergreen configs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.evergreen/config.yml	Updates switch source to fetch SOURCE_REV using a mapping refspec so tag checkouts succeed.
.evergreen/config.in.yml	Mirrors the same switch source fetch refspec fix in the input Evergreen configuration.

[PavelSafronov]

Changes look good, thanks!