(pipelines): Using CodeBuildStep.exportedVariable causes Output for step CodeBuildStep(Produce) not configured. Either the step is not in the pipeline, or this engine does not support Outputs for this step...

### What is the problem?

`CodeBuildStep.exportedVariable` does not seem to work as expected.

### Reproduction Steps

See https://github.com/aws/aws-cdk/pull/19156

```ts
const producer = new cdkp.CodeBuildStep('Produce', {
commands: ['export MY_VAR=hello'],
});

const consumer2 = new cdkp.CodeBuildStep('Consume2', {
buildEnvironment: {
environmentVariables: {
'THE_VAR': {
type: codebuild.BuildEnvironmentVariableType.PLAINTEXT,
value: step.exportedVariable('MY_VAR'),
},
}
},
commands: [
'echo "The variable was: $THE_VAR"',
],
});

```

### What did you expect to happen?

The variable would be referred properly.

### What actually happened?

```text
@aws-cdk/pipelines: Resolution error: Resolution error: Resolution error: Resolution error: Resolution error: Output for step CodeBuildStep(Produce) not configured. Either the step is not in the pipeline, or this engine does not support Outputs for this step...
@aws-cdk/pipelines: Object creation stack:
@aws-cdk/pipelines: at Execute again with CDK_DEBUG=true to capture stack traces.
@aws-cdk/pipelines: Object creation stack:
@aws-cdk/pipelines: at Execute again with CDK_DEBUG=true to capture stack traces..
@aws-cdk/pipelines: 105 | public resolve(_context: IResolveContext) {
@aws-cdk/pipelines: 106 | if (this.resolution === undefined) {
@aws-cdk/pipelines: > 107 | throw new Error(`Output for step ${this.step} not configured. Either the step is not in the pipeline, or this engine does not support Outputs for this step.`);
@aws-cdk/pipelines: | ^
@aws-cdk/pipelines: 108 | }
@aws-cdk/pipelines: 109 | return this.resolution;
@aws-cdk/pipelines: 110 | }

```

### CDK CLI Version

2.15.0 (build 151055e)

### Framework Version

_No response_

### Node.js Version

16

### OS

macOS

### Language

Typescript

### Language Version

4.4.4

### Other information

_No response_

Start with the CodeBuildStep.exportedVariable entry point and the reproduced Produce/Consume2 pipeline configuration. Trace how the exported variable is resolved and how the CodeBuildStep output is configured, then verify the example no longer raises the reported resolution error and that THE_VAR receives the exported value.

Written by the indexing model from the issue text.