guix: use python-minimal (3.9)

Summary:
This further minifies the Guix release build environment.

This is a backport of core#27361
Depends on D15329

Test Plan: contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15330