While you are waiting for SwiftPM to be completed, you can use one of the strategies in the following threads. The first thread is older and generated CMake instructions from SwiftPM on another supported platform before checking out the package on Windows. The second is newer and cross‐compiles out of WSL.
If your intention is mostly just to add a Windows CI job to an existing package using GitHub Actions, then you can simply run a tool of mine with $ workspace refresh continuous‐integration, and it will generate an action configuration for you that includes not only Windows, but also Android and web (which you can then trim however you want). The tool is updated periodically so that it always generates the action scripts according to the current best practice with the least peripheral scaffolding.