fix: pass publicDir in ssr

[bluwy]

We're not passing kit.files.assets to Vite's publicDir in SSR, which I believe is because we don't want to copy those files over in the SSR build?

This PR uses the new (experimental but stable) build.copyPublicDir: false option to prevent the copying instead. I'm not sure if there's anything else I miss, but the tests pass for me locally.

NOTE: I stumbled on this while building the new svelte.dev site as it generates a lot of warnings for missing references to public files, because the option is not set in SSR. This should fix it too.

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