Fix deserializing YAML variables when a build has been imported

What does this MR do?

This merge requests fixes a bug, that caused 500 error on a build page, when a project has been imported. In that case our serialized YAML variables data that we store in the database are no longer symbolized hashes, so we need to symbolize keys in our custom variables serializer.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests added for this bug
• Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#49406