Backend: Optimize CI variable objects
## Summary
When we `concat` variables, we re-create the objects in some places, for example; `Variables::Collection::Item.fabricate`.
We can optimize this.
```bash
curl "https://gitlab.com/api/v4/projects/278964/merge_requests/177414/pipelines?performance_bar=flamegraph" \
-X POST \
-o flamegraph-177414.json \
-H 'accept: application/json' \
-H 'content-type: application/json' \
-H 'cookie: ---' \
-H 'x-csrf-token: ---'
```
issue