Fix board scope update failure when assignee scope already exists

Reference !69423 (comment 666969971)

Reproducible site: https://gitlab.com/groups/gitlab-org/plan-stage/-/boards/3192976?assignee_username=euko

demo	error
Screen_Recording_2021-09-02_at_2.55.31_PM

Error: "Variable $input of type UpdateBoardInput! was provided invalid value for assigneeId (5961404 is not a valid Global ID)"