aws-build-id only set after build runs

The action output `aws-build-id` is only set if the build [completes](https://github.com/aws-actions/aws-codebuild-run-build/blob/main/index.js#L19).


This should probably be set as soon as possible so that it can be used in subsequent actions. We were trying to find a workaround for https://github.com/aws-actions/aws-codebuild-run-build/issues/143 so that in-flight builds can be cancelled.