Issue: SvelteKit headers not set in production build when ssr = false

Reproduction

0. Create SvelteKit skeleton project

1. Set headers in hooks.server.ts

kit-headers-bug/src/hooks.server.ts

Lines 3 to 9 in 6721be2

	export const handle: Handle = async ({ event, resolve }) => {
	event.setHeaders({
	'x-foo': 'bar',
	'cache-control': 'no-store',
	})
	return resolve(event)
	}

2. Disable SSR:

kit-headers-bug/src/routes/+layout.server.ts

Line 1 in 6721be2

export const ssr = false

1. pnpm build
2. pnpm preview
3. Get headers (https -h http://127.0.0.1:4173/)
4. Note the missing headers

NB

1. In this reproduction I used adapter-auto, but behavior is the same with adapter-node
2. The headers are present in dev mode (pnpm dev):