fix(worker): disable manifest plugins in worker build

[bluwy]

Description

When building worker files with Vite plugins, disable the manifest plugins as workers don't need a manifest file. Otherwise the manifest gets emitted into the main Vite build bundle here:

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

Lines 389 to 399 in 273c032

	generateBundle(opts) {
	// @ts-expect-error asset emits are skipped in legacy bundle
	if (opts.__vite_skip_asset_emit__ || isWorker) {
	return
	}
	const workerMap = workerCache.get(config)!
	workerMap.assets.forEach((asset) => {
	this.emitFile(asset)
	workerMap.assets.delete(asset.fileName!)
	})
	},

So when the actual manifest plugin on the main build runs, you would get this warning from Rollup:

The emitted file "manifest.json" overwrites a previously emitted file of the same name.

Additional context

In most case, this is harmless, but there seems to be an fs race condition where the worker manifest gets written down in a SvelteKit build 🤔

It isn't easy to write a test for this too, the logs come from Rollup, and we don't have a setup to track Rollup logs.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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