Pass config.experimental.cpus to export during build

Currently, there is no way of specifying the number of worker threads of
`next export` when run as part of `next build`.

I suggest a sane default should be to just use the same amount of
workers that were used during the build process which currently seems to
be configured through `config.experimental.cpus`.

This setting is already respected in the two other places where
jest-workers are in use: The TerserPlugin and the staticCheckWorkers in
`next build`.

packages/next/build/index.ts

@@ -471,6 +471,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
       sprPages,
       silent: true,
       buildExport: true,
+      threads: config.experimental.cpus,
       pages: combinedPages,
       outdir: path.join(distDir, 'export'),
     }