CodeBuild project environment should support ImagePullCredentialsType

[rli]

https://docs.aws.amazon.com/codebuild/latest/APIReference/API_ProjectEnvironment.html#CodeBuild-Type-ProjectEnvironment-imagePullCredentialsType

"When you use a cross-account or private registry image, you must use SERVICE_ROLE credentials. When you use an AWS CodeBuild curated image, you must use CODEBUILD credentials."

        let buildProject = new codebuild.Project(this, 'build-project', {
            environment: {
                computeType: ComputeType.Large,
                // https://github.com/awslabs/aws-cdk/issues/1325
                buildImage: WindowsBuildImage.fromDockerHub(this.ecrImageUri.stringValue)
            },
            source: new codebuild.CodePipelineSource(),
            buildSpec: 'buildspec.yml'
        });

        (buildProject.node.findChild("Resource") as codebuild.CfnProject)
            .addPropertyOverride("Environment.ImagePullCredentialsType", "SERVICE_ROLE");