Fix tox-docker builds after #29941 #34017
Description
src/VERSION.txt is needed in the Docker container after #29941.
This fixes the failures seen in https://github.com/sagemath/sage/actions/runs/2483481917
and also reported in pypa/setuptools#3389
Component: porting
Author: Matthias Koeppe
Branch/Commit: 58d62f4
Reviewer: François Bissey
Issue created by migration from https://trac.sagemath.org/ticket/34017