more frequent DOWNLOAD_SOURCE failures

@hvo8888 - This error happens when the VPC configured with the CodeBuild project doesn't have connectivity to the configured source. In this case, your project has 4 subnets configured and it seems at least one of these subnets doesn't have access to reach github.com. When more than one subnets are configured with CodeBuild, only one of them is randomly selected for the VPC ingress configuration. You are getting some successful clones, and other times they are failing due to connectivity issues. In order to troubleshoot VPC connectivity, I would suggest going through https://docs.aws.amazon.com/codebuild/latest/userguide/vpc-support.html#troubleshooting-vpc.