Look up NixOS helpers only when trusted system metadata identifies NixOS
and each path component passes ownership and permission checks. Keep the
existing helper search on other distributions.
Detect NoNewPrivs before prompting for administrator credentials, with a
clear error for restricted launchers. Add regression coverage to Linux CI
and append the English fallback message to all language files.
Refs #1911
Mounir IDRASSI
6e63041962