nixos/tests/hydra*: fix eval

To specify distributed build-machines, `nix.distributedBuilds` must be
set to `true` now[1].

[1] 67b6e56391

nixos/tests/hydra/common.nix

@@ -37,6 +37,7 @@
     };
     services.postfix.enable = true;
     nix = {
+      distributedBuilds = true;
       buildMachines = [{
         hostName = "localhost";
         systems = [ system ];