(pipelines): no way to set "synth" action environment variables

[pgarbe]

When using the standardYarnSynth (or npm) action in a CodePipeline it allows me to define environment variables in two places. One is as part of the standardYarnSynth props and the second is part of the environment property. This makes sense as the first should generate the environment variables in the CodePieline action and the second in the CodeBuild project.

But it doesn't matter where I define my variable it always ends as part of the Codebuild project. And this is a problem as some values (like exuction id) can't re resolved within a CodeBuild project but only within a CodePipeline action.

Reproduction Steps

    const synthAction = pipelines.SimpleSynthAction.standardYarnSynth({

      environment: {
        environmentVariables: {
          VERSION_IN: { value: codepipeline.GlobalVariables.executionId },
        },
        buildImage: codebuild.LinuxBuildImage.STANDARD_4_0,
        privileged: true,
      },
      environmentVariables: {
        VERSION_OUT: { value: codepipeline.GlobalVariables.executionId },
      },
      sourceArtifact: sourceArtifact,
      cloudAssemblyArtifact,
      buildCommand: 'yarn build',
    });

What did you expect to happen?

I'd have expected to have "VERSION_IN" as part of the CodeBuild and "VERSION_OUT" as part of the CodePipeline action.

What actually happened?

The sample code above creates a CodeBuild action with two environment variables. But CodeBuild can't resolve these variables.

Environment

• CDK CLI Version : 1.76.0
• Framework Version:
• Node.js Version: 15.3.0
• OS : macOS BigSur (11.0.1)
• Language (Version): TypeScript (3.9.5)

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