tract: Arbitrary file read via unsanitized ONNX external_data `location` (path traversal) on model load in tract-onnx

Summary tract the tract-onnx crate resolves an ONNX tensor's external-data location by joining it onto the model directory without any sanitization. Because location comes from the untrusted .onnx file, a malicious model can make tract open and read an arbitrary local file at load time, with the...