Turbopack: set env in tracing context (#75254)
Part of PACK-4164
~~Set `NODE_ENV` in tracing context~~ For that, see https://github.com/vercel/next.js/pull/75254#issuecomment-2627686346
Set `TURBOPACK` in tracing context
This get rids of some constant build overhead (because we only analyze one instance of react-dom for NFT now, instead of all of the edge/server/client onces).
Otherwise no measureable perf/memory impact
