Fix missing build_script step for custom executor

What does this MR do?

Translates "step_script" to "build_script" stage for the custom executor.

Why was this MR needed?

This is needed as the stage names are in a sense a public API and !1963 (merged) broke it by introducing a new stage, which is not referenced in the documentation: https://docs.gitlab.com/13.1/runner/executors/custom.html#run

What's the best way to test this MR?

Using a custom executor which relies on "build_script" as a stage name, for example the one referenced in #26418 (closed)

The integration test is a good example of that.

What are the relevant issue numbers?

Closes #26418 (closed)