refactor(cli): fix various minor codegen issues

[mrgrain]

Reason for this change

Fixes some small issues that got missed before #31850 was unintentionally merged too early.

Description of changes

• re-add support for CDK_ env variables
• remove square bracktes when commands don't have an alias
• remove extra space in command args
• fixed demandCommand call

Description of how you validated changes

Run via tests, compared manually with cli options before we started to generate them.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 5c89bc7
• Result: SUCCEEDED
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