fix(list): flush output before exiting

[smerrill]

Description

Fixes #3970.

Motivation and Context

Without this code, any code trying to consume the output of lerna list would only get the first 8k.

How Has This Been Tested?

It has not yet been tested other than the simple repro I put into #3970 .

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (change that has absolutely no effect on users)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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✅ Successfully ran 8 targets

• nx run-many --t e2e --parallel=1
• nx run-many -t test --parallel=3 --ci --maxWorkers=2
• nx run-many -t lint --parallel=3
• nx run-many -t build --parallel=3
• nx run integration:integration --ci --maxWorkers=2
• nx-cloud record -- npx nx format:check
• nx prepare-for-e2e e2e-run-task-runner
• nx run-many -t test --parallel=3 --ci --maxWorkers=2 --testTimeout=60000 --testPathIgnorePatterns=save.spec.ts --testPathIgnorePatterns=find-file.spec.ts --testPathIgnorePatterns=chain-class.spec.ts

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[JamesHenry]

Thanks a lot @smerrill!