fix(cli): assetParallelism option in cdk.json is not recognized (#22781)

Version 2.48.0 introduced a flag to enable/disable parallelism when
building and publishing assets (see #22579 ). The flag is supposed to
be set via either environment variable (`CDK_ASSET_PARALLELISM`), command
line argument (`-[no]-asset-parallelism`), or the `cdk.json` file. However,
the last method does not work. We are not passing the option from that
file properly.

----

### All Submissions:

* [X] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/lib/cli.ts

@@ -515,7 +515,7 @@ async function initCommandLine() {
           watch: args.watch,
           traceLogs: args.logs,
           concurrency: args.concurrency,
-          assetParallelism: args.assetParallelism,
+          assetParallelism: configuration.settings.get(['assetParallelism']),
         });
 
       case 'import':