[fix] Prepend backup dir to relative path only and allow absolute in ynh_backup
[fix] Prepend backup dir to relative path only and allow absolute in ynh_backup

YunoHost/yunohost,YunoHost/yunohost,YunoHost/yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost