fix: correctly replace process.env.NODE_ENV

[bluwy]

Description

Fix #8210

Use same logic as definePlugin when resolving the value of process.env.NODE_ENV when replacing.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Looks like there were incorrect tests before that's fixed in the second commit. When testing, process.env.NODE_ENV is test but it was checking based on mode, which is either development or production. I think relying on process.env.NODE_ENV is more correct.