When `openclaw update` regenerates the systemd service file, any user
customizations to ExecStart (e.g. proxychains4 wrapper) are silently
lost. Now the existing unit file is copied to `.bak` before writing
the new one, so users can restore their customizations.
The backup path is printed in the install output so users are aware.
Co-authored-by: echoVic <AkiraVic@outlook.com>