Fix validation of key_path block. (#574)

Follow up to previous change to naming conventions of the Driver config attributes. When a key_path is provided, use the block within the key_path to validate the platform section as well.

src/uwtools/drivers/driver.py

@@ -485,7 +485,7 @@ def _validate(self, schema_file: Optional[Path] = None) -> None:
             validate_internal(
                 schema_name=self.driver_name.replace("_", "-"), config=self._config_intermediate
             )
-        validate_internal(schema_name=STR.platform, config=self.config_full)
+        validate_internal(schema_name=STR.platform, config=self._config_intermediate)
 
     def _write_runscript(self, path: Path, envvars: Optional[dict[str, str]] = None) -> None:
         """