fix: pre-checkout selected PR branch in evergreen workflow

Agent-Logs-Url: https://github.com/githubnext/tsessebe/sessions/018a5ae0-20da-4497-87b7-3f1b2941c9c9

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot