fix(build): avoid warning multiple times

[awxiaoxian2020]

Summary

Fixes #10262.

Problem

Warn 'BLOG_ROOT' not set in .env file multiple times in console.

Solution

Move warning to top level.

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Screenshots

Before

After

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How did you test this change?

run yarn dev.

[awxiaoxian2020]

@fiji-flo PTAL :)