sysconfig variable impermanence in virtual environment when cross-compiling

Bug report

Bug description:

The solution to issue GH-126789 (found in GH-127729 / 70154855cf69) introduced a side effect when sysconfig variables are fetched in a virtual environment with the _PYTHON_PROJECT_BASE environment variable present (on posix platforms). Specifically, every invocation of sysconfig.get_config_vars() and sysconfig.get_config_var() causes the variable cache to be reset and rebuilt from scratch. This can be confirmed by running Python as follows:

virtualenv -p /usr/bin/python3.14 env  # 3.14 or higher

_PYTHON_PROJECT_BASE=/any/path env/bin/python

And then executing:

import sysconfig
sysconfig.get_config_vars()['LIBDIR'] = '/some/new/path'
assert sysconfig.get_config_var('LIBDIR') != '/some/new/path'

Rebuilding the variable cache every time is fairly inefficient and prevents the possibility of tweaking the values (a sometimes necessary evil).

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-150164

使用提供的虚拟环境复现方式和 _PYTHON_PROJECT_BASE 设置，从 sysconfig.get_config_vars() 和 sysconfig.get_config_var() 开始。跟踪缓存为何会在每次调用时重建，添加回归测试覆盖，并验证修改后的值在后续查找中仍然保留。