fix: ensure file names are string-typed

- `$child` was used as an array key earlier. If they are numeric, they
  are automatically converted to ints, leading to type issues later.

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

Author: blizzz

lib/private/Files/Cache/Scanner.php

@@ -516,7 +516,7 @@ private function handleChildren(string $path, $recursive, int $reuse, int $folde
 		$removedChildren = \array_diff(array_keys($existingChildren), $newChildNames);
 		foreach ($removedChildren as $childName) {
 			$child = $path ? $path . '/' . $childName : $childName;
-			$this->removeFromCache($child);
+			$this->removeFromCache((string)$child);
 		}
 		if ($this->useTransactions) {
 			$this->connection->commit();