AWS CDK: Specify Runtime Version for CodeBuild Project

Assuming you have a CodeBuild project and you want to specify the runtime to be Node.js latest, here is your answer:

new PipelineProject(this, `CodeBuildProject`, {
      buildSpec: BuildSpec.fromObject({
          version: '0.2',
          phases: {
            install: {
              'runtime-versions': {
                nodejs: "latest"
              }
            },
            pre_build: {
              commands: [
                "npm install"
              ]
            }
          }
      })
 });