Fallback to using GitHub releases of libsodium if the download site is unavailable

[Gedochao]

Meant to provide a fallback for downloading libsodium, which fails our CI consistently.

download error: Caught java.net.ConnectException (Connection timed out: connect) while downloading https://download.libsodium.org/libsodium/releases/libsodium-1.0.18-stable-msvc.zip
1 targets failed
cli[3.3.5].base-image.staticLibDir os.SubprocessException: Result of C:\Users\runneradmin\AppData\Local\Coursier\cache\arc\https\github.com\coursier\coursier\releases\download\v2.1.24\cs-x86_64-pc-win32.zip\cs-x86_64-pc-win32.exe�: 1

    os.proc.call(ProcessOps.scala:95)
    millbuild.project.settings$CliLaunchers$CliNativeImage.copyLibsodiumStaticTo(settings.sc:239)
    millbuild.project.settings$CliLaunchers$CliNativeImage.$anonfun$staticLibDir$3(settings.sc:[280](https://github.com/VirtusLab/scala-cli/actions/runs/13282281463/job/37086740792#step:5:281))
Error: Process completed with exit code 1.

We've been relying on https://download.libsodium.org/libsodium/releases/, but perhaps we can fall back to https://github.com/jedisct1/libsodium/releases if that's unavailable.