Fix asset browser on subgraph nodes (#6240)

When a widget is linked to a subgraph, the subgraph creates a copy of
the widget. The callback used by the asset browser to update the widget
still refers to the widget that lives inside the subgraph, but at time
of execution, this is overwritten by the unchanged value of the copy.

This is fixed by instead updating the value of the caller. It's a little
hacky, and may need future review.

See also #6237

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6240-Fix-asset-browser-on-subgraph-nodes-2956d73d365081b49bd1cd1a7a254763)
by [Unito](https://www.unito.io)

src/renderer/extensions/vueNodes/widgets/composables/useComboWidget.ts

@@ -76,7 +76,7 @@ const addComboWidget = (
       'asset',
       inputSpec.name,
       displayLabel,
-      async () => {
+      async function (this: IBaseWidget) {
         if (!isAssetWidget(widget)) {
           throw new Error(`Expected asset widget but received ${widget.type}`)
         }
@@ -111,7 +111,7 @@ const addComboWidget = (
             }
 
             const oldValue = widget.value
-            widget.value = validatedFilename.data
+            this.value = validatedFilename.data
             node.onWidgetChanged?.(
               widget.name,
               validatedFilename.data,