CI: add nix for knotd
The extraDepsFor thing takes dependencies from current NixPkgs,
but that doesn't work now due to knot-dns adding libmnl since 3.1.0,
so I extraDeps is added to supply libmnl explicitly in CI
Generally it's a chicken-egg situation between make-archive (the scripts needs dependencies) and finding the dependencies, e.g. from a srcpkg.