And make them fail first.

This commit simplifies greatly the CI logic and improves its
maintability. argopkgs/master becomes the single maintained
authoritative soure of content for CI and packaging.

This commits simplifies the CI setup for this repository, offloading
all complexity to the "argotest" repository.

This commits adds a gitlab-ci integration test target, and the
.integration.nix file, which specifies the exact test version pinning to
use.