unix: Limit named pipe permissions to the current user

Named pipes created using the "pipe://" file access scheme should not be
world-writable or readable. Limit their access to the current user by
creating them with 0600 permissions instead of 0666.

drivers/unix/file_access_unix_pipe.cpp

@@ -65,7 +65,7 @@ Error FileAccessUnixPipe::open_internal(const String &p_path, int p_mode_flags)
 	struct stat st = {};
 	int err = stat(path.utf8().get_data(), &st);
 	if (err) {
-		if (mkfifo(path.utf8().get_data(), 0666) != 0) {
+		if (mkfifo(path.utf8().get_data(), 0600) != 0) {
 			last_error = ERR_FILE_CANT_OPEN;
 			return last_error;
 		}