File polyfills-legacy should not be in strict mode (Plugin legacy)

Describe the bug

For older browsers with IE 11 support generated file polyfills-legacy.[hash].js should not contain use strict directive, but it does.

It is required by polyfill regenerator-runtime/runtime which contains a comment: This module should not be running in strict mode, ... (https://github.com/facebook/regenerator/blob/main/packages/runtime/runtime.js#L736)

Because the built-in file contains use strict, it will result in an error.

Even if I have set

        rollupOptions: {
            output: {
                strict: false // no strict mode, ... but maybe Rollup is wrong, I am not sure
            }
        },

Reproduction

https://stackblitz.com/edit/vite-v5p4en

System Info

System:
    OS: Linux undefined
    CPU: (4) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: Unknown - /bin/jsh
  Binaries:
    Node: 14.16.0 - /usr/local/bin/node
    Yarn: 1.22.10 - /bin/yarn
    npm: 7.17.0 - /bin/npm
  npmPackages:
    @vitejs/plugin-legacy: ^1.6.4 => 1.6.4 
    vite: ^2.6.0 => 2.7.10

Used Package Manager

npm

Logs

No response

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn’t already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[upcccz]

sometimes you need to use generateBundle hook, such as dealing with polyfills-legacy.js

[sapphi-red]

I was not able to reproduce it with IE11.

Also regenerator-runtime/runtime states it works if CSP is not enabled. So if CSP is not set, I think it would work. https://github.com/facebook/regenerator/blob/055b4030219f7cc0af5999ef4431c991697d7ee5/packages/runtime/runtime.js#L739-L748