devel/ruby-build: Update to 20210925

This is a leaf port and worth tool to keep up to date. So should be MFH'ed
to quarterly.

Changes: https://github.com/rbenv/ruby-build/releases/tag/v20210925
Differential Revision: https://reviews.freebsd.org/D32140
MFH: 2021Q3
Approved by: meta (mentor)

(cherry picked from commit 22b087cde88d68ddf322d4cc8d4a6e4db9352f8e)