[next] Add EdgeRuntime provider environment variable to the builder (#…

…8130)

This will enable Next.js to compile and set `vercel` as the edge runtime provider (`EdgeRuntime` global),
which can allow different libraries/customers to have different code running depends on the runtime environment (`edge-runtime` vs `vercel`).

### Related Issues

- vercel/next.js#38331

### 📋 Checklist

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#### Tests

The Next.js feature was not merged yet, so it still can't be tested.

- [ ] The code changed/added as part of this PR has been covered with tests
- [ ] All tests pass locally with `yarn test-unit`

#### Code Review

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packages/next/src/index.ts

@@ -399,6 +399,7 @@ export const build: BuildV2 = async ({
   const env: typeof process.env = { ...spawnOpts.env };
   const memoryToConsume = Math.floor(os.totalmem() / 1024 ** 2) - 128;
   env.NODE_OPTIONS = `--max_old_space_size=${memoryToConsume}`;
+  env.NEXT_EDGE_RUNTIME_PROVIDER = 'vercel';
 
   if (target) {
     // Since version v10.0.8-canary.15 of Next.js the NEXT_PRIVATE_TARGET env