assemble-distro.yml should also run test-all.yml

[fingolfin]

... at the end. And ideally it should do it in a way so that the freshly uploaded package archive is tested. This then can establish the baseline of the test, e.g. for use in PR #196 by @FriedrichRober

So probably that would mean adding back the pkgarchiveurl input (but now made optional) that was removed in 2e075ea, which one could set to the URL of a package distribution tarball. Then the "Download packages" step would be adjusted to use that optional input, if given.