Update Workflow CI stage name

What does this MR do and why?

This MR updates the stage of the workload session to build instead of the default test.

See the context of Ci::Workload at !187094 (merged) . Today it's only used for Duo Agent Platform. It may be used in future. For now the default stage of test is probably not as appropriate as build. Though neither of them are great but in future we can easily make this configurable if needed.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Configure GDK to work with Duo Workflow: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/duo_workflow.md
2. Run the following command to start a workflow:

curl -H "Private-Token: glpat-<redacted>" -XPOST 'http://127.0.0.1:3000/api/v4/ai/duo_workflows/workflows?project_id=1000000&start_workflow=true&goal=Create+a+file+called+hello.sh+that+prints+hello.&source_branch=dev'

3. Open the pipelines http://gdk.test:3000/gitlab-duo/test/-/pipelines locate the pipeline and hover over the stage, it should show build

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #555952 (closed)