Legacy plugin does not respect assetsDir setting

[sqal]

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• Use Vite >=2.0. (1.x is no longer supported)
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Describe the bug

@vitejs/plugin-legacy doesn't respect custom assetsDir setting. In my case, I would like to use 'static' dir, but the plugin emits polyfills-legacy.js chunk in the default 'assets' directory.

Reproduction

import legacy from '@vitejs/plugin-legacy'
import { defineConfig } from 'vite'

export default defineConfig({
  plugins: [
    legacy({
      targets: ['defaults', 'not IE 11']
    })
  ],
  build: {
    assetsDir: 'static'
  }
})

System Info

• vite version: vite@2.0.0-beta.27 / @vitejs/plugin-react-refresh@1.2.0
• Operating System: Windows 10
• Node version: v14.14.0
• Package manager (npm/yarn/pnpm) and version: yarn

Logs (Optional if provided reproduction)

C:\Users\pc\dev\vite-playground>yarn build
building for production...
dist/static/index-legacy.9a045c13.js       0.12kb / brotli: 0.08kb
dist/assets/polyfills-legacy.6a501a48.js   36.51kb / brotli: 12.49kb
dist/static/index.f86dd5ee.js    0.65kb / brotli: 0.32kb
dist/static/index.4bc2bdd2.css   0.29kb / brotli: 0.19kb
dist/index.html                  0.93kb