[v1.x] Backport Improve environment variable handling in unittests (#18424)

[DickJC123]

Description

This is a backport to v1.x of the PR #18424. The original PR built on the work of @larroy, and introduced the with environment(): context and @with_environment() decorator. The PR replaced the ad hoc manipulation of environment variables in unittests and solved the 'read-only env var state' problem of Windows.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

[mxnet-bot]

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