Adds the given build variables to those that already exist.
        :param variables: the build variables to add
        :param overwrite: whether the old variable should be overwritten in the case of a redefinition

def add(self, variables: Dict[str, str], overwrite: bool=False):
        """
        Adds the given build variables to those that already exist.
        :param variables: the build variables to add
        :param overwrite: whether the old variable should be overwritten in the case of a redefinition
        """
        preset_variables = self._project.variables.list()
        preset_variable_keys = [variable.key for variable in preset_variables]

        for key, value in variables.items():
            if key in preset_variable_keys:
                if overwrite:
                    variable = preset_variables[preset_variable_keys.index(key)]
                    variable.value = value
                    variable.save()
            else:
                variable = self._project.variables.create({
                    _VARIABLE_KEY_PROPERTY: key, _VARIABLE_VALUE_PROPERTY: value})
                variable.save()