fix(vite-node): fix missing buildStart

[hi-ogawa]

Description

• Related fix(vite-node): fix buildStart on Vite 6 #7480
• Related fix: await client buildStart on top level buildStart vitejs/vite#19624
• Closes vite-node CLI/binary is completely broken #7666

It looks like css wasn't broken in vite-node since it always executed /@vite/env and this somehow ended up as web transform, which ensures client environment buildStart to be triggered.

It turned out this can still break when users chose --options.transformMode.ssr=".*" #7666. Until vitejs/vite#19624 is released, we can workaround this by grabbing client.pluginContainer.buildStart directly.

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