Include tests that Nixpkgs thinks are required for its Nix updates

[roberth]

          An extra goal - perhaps separate issue? - is to include tests that Nixpkgs thinks are relevant.

We can configure the nix flake to run those, with a little bit of collaboration between the repos.

Originally posted in #9677 (comment)

This can be configured without #9677 to be run on hydra, by extending the existing hydraJobs with those from the Nixpkgs pin, as soon as the nixpkgs repo exposes an interface for us to pull that in. (As to how, perhaps something like NixOS/nixpkgs#176557 (comment) could make this functionality available to all packages that use that code for their tests)