Dummy App generator: Use db var instead of ENV var

[mamhoff]

Summary

A couple of lines before this change, we already normalize ENV['db']. We lose that work by relying on the raw ENV var here.

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[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	v3.4
✅	v4.0
✅	v4.1

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