Actually remove DESTDIR staging for Python packages to eliminate race conditions during Python package installations #32361
Description
Follow-up from #29585, which forgot to include a crucial commit.
Race conditions are still present in parallel builds:
https://github.com/sagemath/sage/runs/3274951485?check_suite_focus=true
[cython-0.29.21] user 1m11.995s
[cython-0.29.21] sys 0m2.867s
[cython-0.29.21] Copying package files from temporary location /sage/local/var/tmp/sage/build/cython-0.29.21/inst to /sage/local
[cython-0.29.21] cp: cannot create directory '/sage/local/./lib/python3.9/site-packages/Cython/__pycache__': File exists
[cython-0.29.21] ************************************************************************
[cython-0.29.21] Error copying files for cython-0.29.21.
[cython-0.29.21] ************************************************************************
CC: @vbraun @jhpalmieri @dimpase
Component: build
Author: Matthias Koeppe
Branch/Commit: 378a034
Reviewer: John Palmieri
Issue created by migration from https://trac.sagemath.org/ticket/32361