Function generate_env_file 

pub fn generate_env_file(
    python_bin: &str,
    rocm_home: &str,
    rocm_lib: &str,
    user_home: &str,
) -> String

Generate a new .mlstack_env file from scratch.

user_home anchors the $HOME-relative paths (e.g. ~/.mlstack/lib-compat) so the file is correct for its owner even when that differs from the live process HOME.