[Perf] Enable environment cache in EngineCore to enable the feature for UniProcExecutor as well

[Jialin]

Purpose

Expand the coverage of the feature. Similar to #26146, we found the environment variable is beneficial to output processing as well.

Test Plan & Test Result

CI Signals

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request refactors the environment variable caching by moving the enable_envs_cache() call from EngineCoreProc to the base class EngineCore. This correctly extends the performance optimization to UniProcExecutor and other non-distributed execution paths. I have reviewed the change and its impact on different executors and initialization sequences. The new placement of the call is safe, occurring after environment-dependent setup and before performance-critical code paths. The change is sound and provides a good performance improvement.

[heheda12345]

Also CC @mgoin

[mgoin]

Nice

[Jialin]

Failed to repro CI test failure locally, so rebase against to confirm if it's a transient issue.

[Jialin]

The CI failure is due to multiple unit tests are running in the same process, so the later test is misusing the environment variable cached in the previous test.

To fix this, introduce disable_envs_cache and invoke in cleanup_dist_env_and_memory which should fully isolate environment variables across 2 tests.