Make workflow custom fields editable (#12063)

Fixes https://github.com/twentyhq/twenty/issues/11989

<img width="1267" alt="Capture d’écran 2025-05-15 à 14 05 15"
src="https://github.com/user-attachments/assets/fbb22f52-2c76-424f-8b8c-fb030fef9fa8"
/>

`isCustom` was not properly set everywhere because it was not mandatory
in `isFieldValueReadOnly`. Removing the default value and adding it to
missing places

packages/twenty-front/src/modules/object-metadata/utils/formatFieldMetadataItemAsFieldDefinition.ts

@@ -39,6 +39,7 @@ export const formatFieldMetadataItemAsFieldDefinition = ({
     options: field.options,
     settings: field.settings,
     isNullable: field.isNullable,
+    isCustom: field.isCustom ?? false,
   };
 
   return {