According to the [official x509 documentation](https://pkg.go.dev/crypto/x509#MarshalPKCS1PrivateKey), the private key generated using `x509.MarshalPKCS1PrivateKey` starts with `-----BEGIN RSA PRIVATE KEY-----` instead of `-----BEGIN PRIVATE KEY-----`. Otherwise, it will not be parsed by most tools (like OpenSSL, [jwt.io](https://jwt.io/), etc.) because it does not conform to the specification.