fix: don't modify config

[sapphi-red]

Description

build.terserOptions is specified but build.minify is not set to use Terser. Note Vite now defaults to use esbuild for minification. If you still prefer Terser, set build.minify to "terser".

This warning was shown even if build.terserOptions was not set.
https://github.com/vitejs/vite/runs/7440261696?check_suite_focus=true#step:13:78

This was happening because of Object.assign(config, resolvedConfig). This PR changes that to Object.assign({}, config, resolvedConfig) to avoid modifing config.

refs #9212

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[sapphi-red]

Surprisingly this PR fixes #9252. 🤔
https://github.com/vitejs/vite-ecosystem-ci/runs/7440371813?check_suite_focus=true

[sapphi-red]

Closing as #9262 supersedes this one.