ci: avoid netlify oom

vitest.config.ts

@@ -22,7 +22,8 @@ export default defineConfig({
   },
   test: {
     globals: true,
-    threads: false,
+    // if not using threads, Netlify goes OOM when generating coverage report
+    threads: process.env.NETLIFY ? true : false,
     setupFiles: 'scripts/setupVitest.ts',
     environmentMatchGlobs: [
       ['packages/{vue,vue-compat,runtime-dom}/**', 'jsdom']