Well, in that case I have a nother convenience snippet:

hydraJobs = builtins.listToAttrs (lib.mapAttrsToList (machine: config:
  lib.nameValuePair machine {
    "x86_64-linux" = config.config.system.build.toplevel;
  }) nixosConfigurations);

Any feedback on how the system can be read from an existing nixosConfiguration would be welcome, to fix the remaining ugly code

And again I think I have an answer to my own question:

hydraJobs = builtins.listToAttrs (lib.mapAttrsToList (machine: config:
  lib.nameValuePair machine {
    "${config.config.nixpkgs.system}" = config.config.system.build.toplevel;
  }) nixosConfigurations);

The system can be read from .#nixosConfigurations.my-machine.config.nixpkgs.system

2 things at play:

1. evaluating staging often means creating a lot of files, and the IO itself can be costly
2. evaluating NixOS means evaluating tests which are computationally costly

Collecting from unknown thread
[...]
error: unexpected EOF reading a line

In reply to @janne.hess:helsinki-systems.de
You need to restart the queue runner from time to time because it sometimes decides it doesn't have to run the queue