Support use of output from synth step as input in later ShellSteps

[giseburt]

I would expect this code to work:

    const pipeline = new GitHubWorkflow(app, 'Pipeline', {
      workflowPath: `${dir}/.github/workflows/deploy.yml`,
      synth: new ShellStep('Build', {
        installCommands: ['yarn'],
        commands: ['yarn build'],
      }),
      jobSettings: { if: 'check on Synthesize and Publish Assets' },
    });

    const stage = new Stage(app, 'MyStage', {
      env: { account: '111111111111', region: 'us-east-1' },
    });

    new Stack(stage, 'MyStack');


    pipeline.addStageWithGitHubOptions(stage, {
      pre: [
        new ShellStep('Extra-Shell-Step', {
          additionalInputs: {
            'cdk.out': pipeline.cloudAssemblyFileSet,
          },
          commands: ['ls -al cdk.out'],
        }),
      ],
    });

The resulting yaml file doesn't work, since the uploaded artifact is named "cdk.out" but the id used as the name from the pipeline.cloudAssemblyFileSet is `"Output":

      Build-Build:
        name: Synthesize
        if: check on Synthesize and Publish Assets
        permissions:
          contents: read
          id-token: none
        runs-on: ubuntu-latest
        needs: []
        env: {}
        steps:
          - name: Checkout
            uses: actions/checkout@v2
          - name: Install
            run: yarn
          - name: Build
            run: yarn build
          - name: Upload cdk.out
            uses: actions/[email protected]
            with:
              name: cdk.out # ← here ⚠️
              path: cdk.out

And later:

      MyStage-Extra-Shell-Step:
        name: Extra-Shell-Step
        if: check on Synthesize and Publish Assets
        permissions:
          contents: read
        runs-on: ubuntu-latest
        needs:
          - Build-Build
        env: {}
        steps:
          - uses: actions/download-artifact@v2
            with:
              name: Output  # ← here ⚠️
              path: cdk.out
          - run: ls -al cdk.out

The name Output is coming from @aws-cdk/pipelines/lib/blueprint/shell-step.ts:

    if (props.primaryOutputDirectory) {
      this._primaryOutputDirectory = props.primaryOutputDirectory;
      const fileSet = new FileSet('Output', this);  // ← here ⚠️
      this.configurePrimaryOutput(fileSet);
      this.outputs.push({ directory: props.primaryOutputDirectory, fileSet });
    }

Perhaps I'm wrong, but it appears that by using this.cloudAssemblyFileSet.id instead of CDKOUT_ARTIFACT here and here this would work as expected.

[giseburt]

I forgot to add that I already have a PR for this teed up as well, just waiting to hear feedback in case it needs adjusted or maybe there's a bigger plan that I'd be invalidating here.