fix race condition in processing css

Author: jonchurch

gulpfile.js

@@ -334,5 +334,6 @@ gulp.task('build-metadata', gulp.parallel('build-appcache', 'minify-xml'))
 // Main build task
 gulp.task('build', gulp.series(
   gulp.parallel('build-headers', 'build-metadata', 'build-redirects'),
-  gulp.parallel('build-css', 'build-html', 'build-images', 'build-js')
+  gulp.parallel('build-html', 'build-js', 'build-images'),
+  'build-css' // run after build-html and build-js, dependent on their output
 ))