Reported from #51 (comment)

The CI failed because OSXCROSS_SDK_URL isn't set (https://circleci.com/gh/prometheus/golang-builder/165). This relates probably to the migration from Circle CI 1.0 to 2.0 and the fact that by default, builds from forked pull requests can't access secrets (see https://circleci.com/docs/2.0/oss/#pass-secrets-to-builds-from-forked-pull-requests).

Not sure what is the fix for this problem.