Fix the CI on this repository

[fingolfin]

It is currently broken.

What is missing is that we need code which produces a "unix release tarball", as that is the input for the scripts here.

Alternatively, if the cron job at https://github.com/gap-system/gap/actions/workflows/release.yml would just upload the tarballs it creates to a fixed URL, we could just use those. That would be useful for other reasons, too.

[ChrisJefferson]

I did an alternative fix, which is also helpful for me when testing the script, but doing this the proper way would be good too.