Problem

The `legacy_mozjs_unit_tests` Evergreen task hardcodes `--define=MONGO_VERSION=9.0.0-alpha-patch-nogitversion` in etc/evergreen_yml_components/tasks/compile_tasks.yml. As the master branch advances, this value can become stale and cause feature-flag-related build failures.

Background

SERVER-134053 and PR #61725 updated this value as an immediate fix after a 9.1 required feature flag exposed the stale version. This follow-up should remove the manual version pin and use the canonical/generated version already used by the build.

Scope of Work

• Replace the hardcoded version in the `legacy_mozjs_unit_tests` task with the canonical MONGO_VERSION source or Evergreen expansion.
• Ensure the legacy MozJS unit tests continue to receive the appropriate version and feature-flag behavior.
• Check related YAML/config-generation paths so future FCV changes do not require another manual version bump.

Acceptance Criteria

• The `legacy_mozjs_unit_tests` task contains no hardcoded release version for MONGO_VERSION.
• The task automatically uses the current canonical/generated MONGO_VERSION as the FCV changes.
• The relevant Evergreen configuration validation or test coverage passes.