How do I force usage of MacOS as a deployment agent machine?



I would like to use CircleCI to build my omnibus package on a MacOS machine (and later upload the artifacts to S3).
On Jenkins I would simply set a label for Mac Minis in my farm, and on travis there is an “os” category you can set.
I don’t seem to find this here.
Also, I want this to apply only for the deployment, the tests can run on any platform (and they already do).