chore: kebab-case post-processing flag

[jasonbarry]

Summary

This PR changes the post-processing flag from snake_case to kebab-case to match the convention of deploy context values (e.g. branch-deploy and deploy-preview). Our API supports both snake and kebab, but moving them to kebab in our CLI will make them more consistent when showing help commands like netlify env:list --help.

Changes to open-api forthcoming.

https://github.com/netlify/pillar-workflow/issues/808

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[github-actions]

📊 Benchmark results

Comparing with 791a398

Package size: 241 MB

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    T-1      T    

Legend

• T-1 (791a398): 241 MB
• T (current commit): 241 MB

[eduardoboucas]

LGTM