nixos/hydra: fix test

We ship `https://cache.nixos.org` as binary cache by default which
automatically substitutes the test derivation used inside the Hydra
test. However it needs to be built locally to confirm that
`hydra-queue-runner` works properly.

Also inherited the platform name for the test derivation from `system`
to ensure that the build can be tested on each supported platform.

ZHF #68361

nixos/tests/hydra/default.nix

@@ -8,8 +8,10 @@ let
   trivialJob = pkgs.writeTextDir "trivial.nix" ''
    { trivial = builtins.derivation {
        name = "trivial";
-       system = "x86_64-linux";
+       system = "${system}";
        builder = "/bin/sh";
+       allowSubstitutes = false;
+       preferLocalBuild = true;
        args = ["-c" "echo success > $out; exit 0"];
      };
    }
@@ -57,7 +59,7 @@ let
             nix = {
               buildMachines = [{
                 hostName = "localhost";
-                systems = [ "x86_64-linux" ];
+                systems = [ system ];
               }];
 
               binaryCaches = [];