Add variable for epic sync test

What does this MR do and why?

Adds a variable needed for testing legacy epic to work item migration sync.

Related to gitlab!154469 (merged), gitlab-org/quality/quality-engineering/team-tasks#2651 (closed), and https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/8222

Tested here: https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/14414030 using gitlab-org/quality/pipeline-common!429 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Commits include Changelog: trailer
• I have evaluated the MR acceptance checklist for this MR.