Function build_hosts 

pub(crate) fn build_hosts(
    config: &ReleaseConfig,
    hosts: &[(String, HostKind)],
    resolved: &FleetResolved,
    current_commit: Option<&str>,
) -> Result<BTreeMap<String, PathBuf>>

Sequential build. No pin or pin.commit == current_commit -> local build path; non-current commit -> flake-ref build via <pin_source_url>?rev=<commit>. Cross-platform builds rely on the operator’s nix.buildMachines. Failures abort before any push.