Verify `GithubCiLayer.build_setup` docs

In `dist::config::v1::ci::github::GithubCiLayer`, the `permissions` and `build_setup` fields have the same documentation, which seems wrong:

https://github.com/axodotdev/cargo-dist/blob/d40391dd0bfb3e2e6c40c1cecd844fe7694eaae7/cargo-dist/src/config/v1/ci/github.rs#L24-L30

It's unclear to me what the documentation for `build_setup` is _supposed_ to be.

Read cargo-dist/src/config/v1/ci/github.rs around lines 24-30 and inspect the surrounding GithubCiLayer fields and related configuration documentation. Determine the intended meaning of build_setup from the nearby code or established documentation, then update its description so it is distinct from permissions. Done means the two fields no longer share incorrect documentation.

Written by the indexing model from the issue text.