How to skip a build stage in AWS CodeBuild - batch/build-graph

Question

I have a codebuild in aws codebuild. For instance, the buildspec.yml as below:

batch:
 fast-fail: false
 build-graph:
  - identifier: build1
    env:
      variables:
        BUILD_ID: build1
     ignore-failure: false
  - identifier: build2
    buildspec: build2.yml
    env:
      variables:
        BUILD_ID: build2
    depend-on:
      - build1
  - identifier: build3
    env:
      variables:
        BUILD_ID: build3
    depend-on:
      - build2

From the CodePipeline, I am passing a variable - RunBuild1. Depending upon the value of the variable, I want to control the run for build1

What I exactly want is, when I have the variable - RunBuild1 is true, then run build1, if the variable is false skip build1.

How can I do this?

Thanks in advance.