No worries.... I expect them to ask you about your work status during the interview. If they can't hire you because of your F1 status, they will advise you accordingly. From what I have seen before, they have hired many full time post-docs on their OPT status. I think the citizenship requirement is for regular job, not for fixed term contract jobs. Good luck for your interview.

* Disclaimer: I am not lawyer and take this advice on your own risk.