Comment by mfenniak

Forgejo is committed to using exclusively Free Software for it's own project development. Windows and Mac versions of the Forgejo Runner are built in the project's CI system as a minimal check to ensure platform compatibility, but due to the project's commitment, the project doesn't do integration testing on these platform. And therefore doesn't distribute untested software.

A contributor maintains a tested re-release of Forgejo Runner for Windows: https://github.com/Crown0815/Forgejo-runner-windows-builder

But, pull it down and build it, and it will work.