The ConfigBuilderGenerator is instantiated in three places, two of which use kernel.build_dir as the output dir, and one which uses kernel.cache_dir:

• symfony/src/Symfony/Component/HttpKernel/Kernel.php

  Line 736 in 29f427f

  new PhpFileLoader($container, $locator, $env, class_exists(ConfigBuilderGenerator::class) ? new ConfigBuilderGenerator($this->getBuildDir()) : null),

• symfony/src/Symfony/Component/DependencyInjection/Extension/ExtensionTrait.php

  Lines 55 to 61 in 29f427f

  	$buildDir = $container->getParameter('kernel.build_dir');
  	$locator = new FileLocator();
  	$resolver = new LoaderResolver([
  	new XmlFileLoader($container, $locator, $env),
  	new YamlFileLoader($container, $locator, $env),
  	new IniFileLoader($container, $locator, $env),
  	new PhpFileLoader($container, $locator, $env, new ConfigBuilderGenerator($buildDir)),

• symfony/src/Symfony/Bundle/FrameworkBundle/CacheWarmer/ConfigBuilderCacheWarmer.php

  Lines 43 to 45 in 29f427f

  	public function warmUp(string $cacheDir): array
  	{
  	$generator = new ConfigBuilderGenerator($cacheDir);

This PR fixes this inconsistency so that kernel.build_dir is always used.