Function install_log_path 

pub fn install_log_path(
    logs_root: &Path,
    plugin_name: &str,
    now_rfc3339: &str,
) -> PathBuf

Build the per-install log path. Caller is expected to create the parent directory before opening it. Format:

{logs_root}/install/{plugin}-{rfc3339}.log

where rfc3339 has colons replaced with - so the filename is safe on Windows (and grep-friendly).