Describe the bug
An issue with the serialization of 'nonstd_instruments' in processerspec.py that causes ProtocolData with experiment designs generated from a processor spec to not write.

To Reproduce
Steps to reproduce the behavior:

1. Create a processor spec
2. Create a target model from the processor spec
3. Add the quantum instrument to the target model
   target_model[('Iz', 0)] = pygsti.modelmembers.instruments.Instrument({'p0': Q0 'p1': Q1})
4. Generate an experiment design from the target model
5. Generate data
6. Create a ProtocolData & attempt to save:

Expected behavior
The serialization should work correctly, allowing me to save ProtocolData with experiment designs generated from a processor spec to not write.

Environment (please complete the following information):

• pyGSTi version feature-qiskit-mcm
• python version 3.10