coerce work pool env values to str

[zzstoatzz]

when a value like {"bar": 42} (valid JSON) is set on a workpool's env, the changes in pydantic 2 made it so that BaseJobConfiguration (which has env: Dict[str, str | None]) was failing on 42 with validation errors

all env values should be strings, so this PR adds a model_validator to coerce any non None values that are not strings when we construct a job config object

[desertaxle]

LGTM!