Have I to do something in particular in order to make remote builder for a different system work on Hydra?

nix = {
  buildMachines = [
      {
        hostName = "rock5b.fleet";
        system = "aarch64-linux";
        maxJobs = 6;
        speedFactor = 2;
        supportedFeatures = ["nixos-test" "benchmark" "big-parallel" "kvm"];
        mandatoryFeatures = [];
        sshKey = "/etc/ssh/ssh_host_ed25519_key";
      }
    ];
    distributedBuilds = true;
};

I've this on my machine (x86_64-linux) and I can confirm that I can remote build using nix build <drv>, in Hydra UI "machines" page I see both localhost and ssh://rock5b (aarch64-linux). I've also configured ssh access for the hydra user, so I can access the remote builder using ssh being the hydra user (simply added the ssh keys to /var/lib/hydra/.ssh). Still hydra doesn't build aarch64-linux derivations and gives me "unsupported system type" :(

Am I missing something?

Logs don't help, if I manually restart the build from the UI I get this:

Apr 14 09:57:07 mothership hydra-queue-runner[929743]: aborting unsupported build step '/nix/store/cf0kfmf12cqrcvjjsp48zbzp20nw1q5q-treefmt.toml.drv' (type 'aarch64-linux')
Apr 14 09:57:07 mothership hydra-queue-runner[929743]: marking build 371 as failed