Download xz from GitHub

[radarhere]

The primary maintainer of xz suggests that it is better to download the package from GitHub than from tukaani.org

tukaani-project/xz#170 (comment)

You might wish to use direct GitHub (or Sourceforge URL) for the downloads, especially if there are many users who build from source with the build script which results in a large number of downloads. I do keep the tukaani.org links working, but they may (or may not) redirect to GitHub to reduce bandwidth usage.

GitHub URLs are like this: https://github.com/tukaani-project/xz/releases/download/v$XZ_VERSION/xz-$XZ_VERSION.tar.gz

[radarhere]

I noticed that libjpeg-turbo was failing in the test job - https://github.com/multi-build/multibuild/actions/runs/14209789094/job/39814741969

So I've updated the libjpeg-turbo version to fix that.

[mattip]

Thanks