fix(css): reset render cache on renderStart

[bluwy]

Description

Fix unocss fail in ecosystem-ci, since #13974 that revealed a bug in our code.

Additional context

Unocss has a test in lib mode with multiple outputs, each output is rendered once. The css-post plugin has caches for the render hooks, e.g. renderChunk and generateBundle, the caches are scoped to each render (aka each output).

However, we reset our cache in buildStart, that means each render share the same cache, which is incorrect. This PR changes to renderStart instead.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

Nice catch! This may also be an issue in the manifest plugin:

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

Line 31 in 80c6608

buildStart() {

[bluwy]

I think the manifest plugin is intentional because it wants to output the manifest only on the final output