flowey: update `download_gh_release` to include curl-based "fast path" for public artifacts

[daprilik]

In the brief window where OpenVMM was on github, but was still closed-source, we used the gh CLI in order to properly authenticate with private repos in order to download various artifacts (e.g: the mu_msvm UEFI package, via xflowey restore-packages and xflowey build-igvm).

Now that we are open-source, we should be able to sidestep the gh CLI entirely, and pull public github releases down using curl.

This is nice, as it avoids needing uses to log-in to the gh CLI tool, making cargo xflowey restore-packages a truly "hands off" dev-ex.