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.