[fix] Backup var unset

scripts/backup

@@ -10,11 +10,11 @@ app=$YNH_APP_INSTANCE_NAME
 . /usr/share/yunohost/helpers
 
 # Backup sources & data
-ynh_backup "/opt/$APP" "sources"
+ynh_backup "/opt/$app" "sources"
 
 # Copy NGINX configuration
 domain=$(ynh_app_setting_get "$app" domain)
 ynh_backup "/etc/nginx/conf.d/${domain}.d/${app}.conf" "nginx.conf"
 
 # Copy Gunicorn pool configuration
-ynh_backup "/etc/systemd/system/$APP.service" "gunicorn.service"
+ynh_backup "/etc/systemd/system/$app.service" "gunicorn.service"