Build environment variables suitable for passing to the Model.

def _build_environ(self) -> Dict[str, Optional[str]]:
        """
        Build environment variables suitable for passing to the Model.
        """
        d: Dict[str, Optional[str]] = {}

        if self.__config__.case_insensitive:
            env_vars = {k.lower(): v for k, v in os.environ.items()}
        else:
            env_vars = cast(Dict[str, str], os.environ)

        for field in self.__fields__.values():
            if field.has_alias:
                env_name = field.alias
            else:
                env_name = self.__config__.env_prefix + field.name.upper()

            env_name_ = env_name.lower() if self.__config__.case_insensitive else env_name
            env_val = env_vars.get(env_name_, None)

            if env_val:
                if field.is_complex():
                    try:
                        env_val = json.loads(env_val)
                    except ValueError as e:
                        raise SettingsError(f'error parsing JSON for "{env_name}"') from e
                d[field.alias] = env_val
        return d