fix(legacy): don't force set build.target when renderLegacyChunks=false (fixes #10201)

[sapphi-red]

Description

#10072 force set build.target, but it's not necessary when renderLegacyChunks=false.
For example, when using plugin-legacy for only generating polyfills (#10201 (comment)).

fixes #10201

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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