Fix is_public during upgrade

scripts/upgrade

@@ -25,12 +25,7 @@ db_pwd=$(ynh_app_setting_get --app=$app --key=mysqlpwd)
 language=$(ynh_app_setting_get --app=$app --key=language)
 
 trustedversion="4.4-1+stretch"
-if [ ynh_permission_has_user --permission=main --user=visitors ]
-then
-	is_public=0
-else
-	is_public=1
-fi
+is_public=$(ynh_permission_has_user --permission=main --user=visitors)
 
 #=================================================
 # CHECK VERSION