fix: Use a separate cached PostCSS config for each CSS plugin instantiation

[fwouts]

Description

The PostCSS config is currently cached once for the entire lifetime of the Node process. This assumes that, for a given Node process, Vite only runs on a single project. That assumption breaks for example when using Vite's API to preview different projects, each with their own PostCSS config.

This PR moves the cachedPostcssConfig variable to the cssPlugin function scope, so that each Vite server has its own independently cached value (enabling multiple projects to run in parallel).

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.

[fwouts]

Closing because #3906 already fixes this :)

[y1d7ng]

Closing because #3906 already fixes this :)

I'm sorry. I commented at first, but then I found out that my solution might have an issue when vite serve, so I deleted the comment to confirm the problem.

[fwouts]

No worries at all, either way we're better off with only one PR! Feel free to use my code if that helps too.

[y1d7ng]

No worries at all, either way we're better off with only one PR! Feel free to use my code if that helps too.

Thanks a lot