feature #50951 [FrameworkBundle] Support APP_BUILD_DIR (ro0NL)

This PR was squashed before being merged into the 6.4 branch.

Discussion
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[FrameworkBundle] Support APP_BUILD_DIR

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->
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follows `APP_CACHE_DIR`

Commits
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664af6d40d [FrameworkBundle] Support APP_BUILD_DIR

Author: fabpot

Kernel/MicroKernelTrait.php

@@ -114,6 +114,15 @@ public function getCacheDir(): string
         return parent::getCacheDir();
     }
 
+    public function getBuildDir(): string
+    {
+        if (isset($_SERVER['APP_BUILD_DIR'])) {
+            return $_SERVER['APP_BUILD_DIR'].'/'.$this->environment;
+        }
+
+        return parent::getBuildDir();
+    }
+
     public function getLogDir(): string
     {
         return $_SERVER['APP_LOG_DIR'] ?? parent::getLogDir();