Use `EnvironmentVarGuard` in tests instead of manually messing with `os.environ`

Feature or enhancement

Proposal:

Some tests manually tweak os.environ. There is new way to do that with EnvironmentVarGuard. Identify all places and update.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-128439
• gh-128476
• gh-128522
• gh-130002
• gh-131870

在 CPython 測試套件中搜尋手動修改 os.environ 的測試，然後將這些用法與 EnvironmentVarGuard 進行比較。開始之前先檢視連結的 PR（gh-128439、gh-128476、gh-128522、gh-130002 和 gh-131870）；當所有適用的手動環境變更都已遷移且受影響的測試通過時，工作就完成了。