Assume correct EC2 role in search index test task.

Author: matthewdale

.evergreen/config.yml

@@ -2078,6 +2078,7 @@ task_groups:
   - name: test-search-index-task-group
     setup_group:
       - func: setup-system
+      - func: assume-test-secrets-ec2-role
       - command: subprocess.exec
         params:
           working_dir: src/go.mongodb.org/mongo-driver