Fetch MR source branch before danger dry_run

What does this MR do?

The danger dry_run command does seem to not know how to fetch --head $CI_MERGE_REQUEST_SOURCE_BRANCH_SHA on squash merges with merge trains. See https://gitlab.com/gitlab-org/cli/-/jobs/5377339769 for an example.

I think what’s happening is that the local branch creation failed in this step because the commit wasn’t present, but that this check is not working as intended, so we get a cryptic failure much later on.

The error about danger_h is because it's the first 8 characters of danger_head being formatted as a short SHA when raising the error.

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.