CI: Generate downloadable tarballs as part of NUT CI farm builds

[jimklimov]

Previously this was done by buildbot, but it is planned for deprecation.

[jimklimov]

See https://javahelps.com/manage-github-artifact-storage-quota for hints about making a github action for this to keep a few recent builds afloat and stay within quota.

Maybe this would benefit from a pre-defined Docker container to have all the tools and prerequisites installed once.

[jimklimov]

This may be useful also for #1209.