fix(vite-plugin): return css in production (#729)

* fix(vite-plugin): return css in production

* chore: add changeset

Author: danielroe

.changeset/fuzzy-flowers-fold.md

@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/vite-plugin': patch
+---
+
+Generate CSS and not HMR code if vite server is present in production mode

packages/vite-plugin/src/index.ts

@@ -96,7 +96,7 @@ export function vanillaExtractPlugin({
         return;
       }
 
-      if (!server) {
+      if (!server || server.config.isProduction) {
         return css;
       }