Visual Studio Online (VSO) has a new build system that will eventually replace the old XAML based system.
VSO now also lets you perform builds using an on-prem build agent (or one hosted in a VM on Azure etc.).
I haven not been able to find information about this, but I assume that using on-prem agents will not count against the VSO build minutes. I assume that running builds are “free” contrary to using hosted agents, where there are 60 minutes included in VSO, and then you have to pay for additional.