I’m not familiar with PackageCompiler internals, but my guess is that you don’t find anything about cross-compilation in the docs simply because you can’t: julia isn’t a cross-compiler at the moment. JULIA_CPU_TARGET is only to choose a specific microarchitecture within the host architecture, but you’ll likely be unable to target a different architecture, and a different operating system.
If your code is on GitHub, perhaps you can use GitHub Actions CI/CD service to build your application.