Optimize loading workbox::injectManifest

[userquin]

If the user disables injection point, there is no need to load workbox here

vite-plugin-pwa/src/modules.ts

Lines 102 to 105 in dd3e003

	const [workbox, buildSW] = await Promise.all([
	loadWorkboxBuild(),
	import('./vite-build').then(({ buildSW }) => buildSW),
	])

We can just provide the promise to the custom vite build and await when there is injection point.