debian/rules: set correct path for zsh completion

• ref: https://salsa.debian.org/debian/zsh/-/blob/debian/debian/README.Debian?ref_type=heads#L73-89
• ref: zsh@bf0a44a8

% zsh --norcs -c 'print -l $fpath |grep -E "site-functions|vendor"'
/usr/local/share/zsh/site-functions
/usr/share/zsh/vendor-functions
/usr/share/zsh/vendor-completions