Fix prebuild race between libpam and libtacplus.

Fixes: 21850106fdda
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans, emaste
Differential Revision: https://reviews.freebsd.org/D40702

(cherry picked from commit bd9049eeff458fc7ee256a891ba285519c6ce550)