fix(css): cachedPostcssConfig reused for multiple builds

[y1d7ng]

Description

fix #3902

Additional context

const { build } = require("vite");

(async () => {
  await build({
    ....
    css: {
      postcss: PostcssConfig1
    }
  })
  await build({
    ....
    css: {
      postcss: PostcssConfig2
    }
  })
})();

PostcssConfig1is reused, PostcssConfig2is invalid. caused by cachedPostcssConfig

vite/packages/vite/src/node/plugins/css.ts

Lines 724 to 731 in f623ba3

	let cachedPostcssConfig: PostCSSConfigResult | null | undefined
	async function resolvePostcssConfig(
	config: ResolvedConfig
	): Promise<PostCSSConfigResult | null> {
	if (cachedPostcssConfig !== undefined) {
	return cachedPostcssConfig
	}

---

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]

@OneNail do you need help to write up tests?

[patak-dev]

@fwouts that would be great! It would be nice if you could PR some tests for this targeting OneNail fork so we can merge this together with them

[y1d7ng]

@OneNail do you need help to write up tests?

@fwouts That's great, thank you very much.

[fwouts]

FYI I've added tests in y1d7ng#1, please review when either of you have time :)

[y1d7ng]

@fwouts helped to add the test, @patak-js @Shinigami92 please review this pr when either of you have time, thanks.