Fix potentially corrupt merged CSS files.

Create temporary files in the same static directory as the target file
to ensure that both files are on the same filesystem and thus the call
to rename will be atomic.

This resolves #29172.

Author: Peter O'Callaghan

lib/internal/Magento/Framework/View/Asset/MergeStrategy/Direct.php

@@ -61,9 +61,8 @@ public function merge(array $assetsToMerge, Asset\LocalInterface $resultAsset)
         $filePath = $resultAsset->getPath();
         $tmpFilePath = $filePath . $this->mathRandom->getUniqueHash('_');
         $staticDir = $this->filesystem->getDirectoryWrite(DirectoryList::STATIC_VIEW);
-        $tmpDir = $this->filesystem->getDirectoryWrite(DirectoryList::TMP);
-        $tmpDir->writeFile($tmpFilePath, $mergedContent);
-        $tmpDir->renameFile($tmpFilePath, $filePath, $staticDir);
+        $staticDir->writeFile($tmpFilePath, $mergedContent);
+        $staticDir->renameFile($tmpFilePath, $filePath, $staticDir);
     }
 
     /**