fix: vite build --watch not emitting anything (#2108)

There was a regression introduced by b7a6687
where vite projects built using `vite build --watch` will only generate once.
Subsequent generations will just leave the placeholder.

See #963

packages/vite/src/modes/global/build.ts

@@ -93,8 +93,10 @@ export function GlobalModeBuildPlugin(ctx: UnocssPluginContext<VitePluginConfig>
       },
       load(id) {
         const layer = resolveLayer(getPath(id))
-        if (layer)
+        if (layer) {
+          vfsLayers.add(layer)
           return getLayerPlaceholder(layer)
+        }
       },
       moduleParsed({ id, importedIds }) {
         if (!layerImporterMap.has(id))