fix: esbuild minification and renderLegacyChunks false

[DreierF]

Description

When using vite 2.6.0-beta.2 together with plugin-legacy and renderLegacyChunks set to false the build fails with

Error: Can't use esbuild as the minifier when targeting legacy browsers because esbuild minification is not legacy safe.

even though no legacy code is generated that might be incompatible with esbuild.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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