Verify GithubCiLayer.build_setup docs

[duckinator]

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

cargo-dist/cargo-dist/src/config/v1/ci/github.rs

Lines 24 to 30 in d40391d

	/// Custom permissions for jobs
	#[serde(skip_serializing_if = "Option::is_none")]
	pub permissions: Option<SortedMap<String, GithubPermissionMap>>,
	/// Custom permissions for jobs
	#[serde(skip_serializing_if = "Option::is_none")]
	pub build_setup: Option<String>,

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

[duckinator]

If this is updated, apply_ci_github() should be updated as well.