Use GitHub hosted Runners.

[JulianGro]

Apparently, on Conan, our disk space usage went down quite a lot, to the point where we can use the free Runners hosted by GitHub again. (Thought this might be because of how we provide custom images, instead of using GitHub's Runner images.) It's not exactly a huge monetary saving (10-20€ per month), but it saves me maintenance work and is more reliable that hosting our own GitHub Actions runners.