Function build_flake_attr 

pub fn build_flake_attr(
    flake_attr: &str,
    machines_file: Option<&str>,
) -> Result<String>

Build any flake attribute and return its store path.

This is the generic build primitive — consortium-ansible uses it for ansibleEnvs.{name}, consortium-slurm for slurmEnvs.{name}, etc.