bpo-45022: Pin current libffi build to fixed version in preparation for upcoming update (GH-27982)

Also improve the build script for libffi, which is not used as part of the regular build.
(cherry picked from commit 969ae7f7356584e30667b4e490ffa2ffa1810429)

Co-authored-by: Steve Dower <steve.dower@python.org>