Function build_pinned 

pub(crate) fn build_pinned(
    pin_source_url: &str,
    commit: &str,
    kind: HostKind,
    host: &str,
) -> Result<String>

Build via flake-ref at a different commit. The url?rev=<commit> form lets Nix handle checkout + caching; we don’t manage a worktree ourselves.