feat: expose env var in cust training class run func args

[morgandu]

Fixes #<b/186513106> 🦕

[sasha-gitg]

I think this is failing because, unlike model_to_upload, training_task_inputs is a struct.Value:

https://github.com/googleapis/python-aiplatform/blob/master/google/cloud/aiplatform_v1/types/training_pipeline.py#L161

Try passing in a dictionary with the same schema. Something like::

spec["pythonPackageSpec"]["env"] = [
  {'name':key, 'value': value} for key, value in environment_variables.items()]

[sasha-gitg]

Thanks Morgan!