[Filesystem] Fix dumpFile stat failed error hitting custom handler

Since #54471, dumpFile will trigger a `fileperms(): stat failed`
error when writing to a filename that does not yet exist. This
was silenced from PHP's default handler with the `@` operator.

However, the error is still passed to any custom handler that the
application has registered, and can therefore cause exceptions or
spurious logging depending on the implementation of the handler.

The better solution, which is consistent with all other calls to
native functions in this class, would be to use `self::box` to
catch and ignore the potential error so that it never leaks
outside this class.

Author: acoulton

Filesystem.php

@@ -691,7 +691,7 @@ public function dumpFile(string $filename, $content)
                 throw new IOException(sprintf('Failed to write file "%s": ', $filename).self::$lastError, 0, null, $filename);
             }
 
-            self::box('chmod', $tmpFile, @fileperms($filename) ?: 0666 & ~umask());
+            self::box('chmod', $tmpFile, self::box('fileperms', $filename) ?: 0666 & ~umask());
 
             $this->rename($tmpFile, $filename, true);
         } finally {