(pipelines): Adding a ManualApprovalStep to a Wave

It seems that the following code does not really add the `ManualApprovalStep` before the build action. What would be the proper way to do this with the new CDK Pipelines?

```ts
pipeline.addWave('CustomWave', {
pre: [new pipelines.ManualApprovalStep('ManualApproval')],
post: [
new pipelines.CodeBuildStep('CustomWaveAction', {
projectName: 'CustomWaveActionBuild',
commands: [
// add rest of the commands here..
],
})],
});

```

### Environment

- **CDK CLI Version :** 2.0.0-rc.17 (build fb5dc58)
- **Framework Version:** 2.0.0-rc.17 (build fb5dc58)
- **Node.js Version:** 14
- **OS :** macOS
- **Language (Version):** all

### Other

---

This is :bug: Bug Report

Start by reproducing the `pipeline.addWave` example with `ManualApprovalStep` in `pre` and `CodeBuildStep` in `post`, using the reported CDK Pipelines versions. Trace how `addWave`, `pre`, and `post` are translated into pipeline stages; done means the manual approval gates the build action as intended.

Written by the indexing model from the issue text.