Function setup_auth 

pub(crate) fn setup_auth(
    cmd: &mut Command,
    fds: &mut CmdFds,
    sysroot: &Dir,
) -> Result<()>

Set up REGISTRY_AUTH_FILE on a command, passing the bootc/ostree auth file via an anonymous tmpfile fd.

If no bootc-owned auth is configured, an empty {} is passed to prevent podman from falling back to user-owned auth paths.