@johantell OK, we have confirmed the cause and will get a more permanent fix in soon. In the mean time you should be able to get unblocked by visiting the legacy settings page that is at: https://circleci.com/gh/[yourorg]/[yourproject]/edit#build-environment and setting the macOS option to “off” (I will send you email with a link to your specific project, but posting this here for others.
This should now be fixed (projects will no longer be errantly flagged to run on macs based on their code at the time the project is added). Please let us know if you see this behavior on any new projects. On old projects you can use the fix above.