perf(ngcc): reduce maximum worker count

Recent optimizations to ngcc have significantly reduced the total time
it takes to process `node_modules`, to such extend that sharding across
multiple processes has become less effective. Previously, running
ngcc asynchronously would allow for up to 8 workers to be allocated,
however these workers have to repeat work that could otherwise be shared.
Because ngcc is now able to reuse more shared computations, the overhead
of multiple workers is increased and therefore becomes less effective.
As an additional benefit, having fewer workers requires less memory and
less startup time.

To give an idea, using the following test setup:

```bash
npx @angular/cli new perf-test
cd perf-test
yarn ng add @angular/material
./node_modules/.bin/ngcc --properties es2015 module main \
  --first-only --create-ivy-entry-points
```

We observe the following figures on CI:

|                   | 10.1.1    | PR angular#38840 |
| ----------------- | --------- | --------- |
| Sync              | 85s       | 25s       |
| Async (8 workers) | 22s       | 16s       |
| Async (4 workers) | -         | 11s       |

packages/compiler-cli/ngcc/src/main.ts

@@ -162,8 +162,8 @@ function getExecutor(
     const {retryAttempts, retryDelay} = config.getLockingConfig();
     const locker = new AsyncLocker(lockFile, logger, retryDelay, retryAttempts);
     if (inParallel) {
-      // Execute in parallel. Use up to 8 CPU cores for workers, always reserving one for master.
-      const workerCount = Math.min(8, os.cpus().length - 1);
+      // Execute in parallel. Use up to 4 CPU cores for workers, always reserving one for master.
+      const workerCount = Math.min(4, os.cpus().length - 1);
       return new ClusterExecutor(
           workerCount, fileSystem, logger, fileWriter, pkgJsonUpdater, locker,
           createTaskCompletedCallback);