Hi, I'm the author of the repo.
I use electron-builder [0] to build my Electron app. IIRC there are a few CLI flags to build for different operating systems without needing to run it in the target OS. So you don't even need to use GitHub Actions.
I was unable to build a Snap Store package for arm on an amd64 machine. Canonical provides their own remote build service, but proprietary projects can't use it yet.
In order to build a package for distribution on the Mac App Store I have to use macOS with XCode installed and signing certificates stored in the system keychain. Uploading the package to the app store also must be done with macOS-specific tools. At least it is possible to build multi-arch packages, so I don't need two separate machines as it is the case with snaps.
Windows 10 with Windows SDK is required to build an appx package for distribution on the Microsoft Store.