test-all.yml: how to "best" download packages

[fingolfin]

Not sure how to best download packages in test-all.yml:

1. using download_packages.py has the advantage that we really test the packages in the commit this action runs on
2. downloading a packages.tar.gz from a given ensures we really test the tarball we later offer to users; but it requires a download URL for such a package

In practice, I envision that this is run from either ...

1. a PR workflow (to test whether a PR is ready to be merged),
2. upon a push to the main branch (to test a packages.tar.gz we just assembled),
3. as part of a scheduled cron job, to test for regressions introduced in GAP master

For the first scenario, it makes sense to use download_packages.py; for the other two scenarios, I'd say the second approach makes more sense. So perhaps both should be supported, by turning the pkgarchiveurl into an optional argument?

Originally posted by @fingolfin in #78 (comment)