nixos/nix-daemon: assert system or systems for buildMachines.

Commit 5395397f removed the assertions from the buildMachines to
ensure that either system or systems is set for each buildmachine.

This patch re-implements those assertions.

The symptom is that if both system and systems are omitted, then the
/etc/machines file has the wrong number of columns and any attempt to
run a `nix` operation that has to perform a build will fail with a
`strtoull` exception.

nixos/modules/services/misc/nix-daemon.nix

@@ -533,6 +533,22 @@ in
             + "\n"
           ) cfg.buildMachines;
       };
+    assertions =
+      let badMachine = m: m.system == null && m.systems == [];
+      in [
+        {
+          assertion = !(builtins.any badMachine cfg.buildMachines);
+          message = ''
+            At least one system type (via <varname>system</varname> or
+              <varname>systems</varname>) must be set for every build machine.
+              Invalid machine specifications:
+          '' + "      " +
+          (builtins.concatStringsSep "\n      "
+            (builtins.map (m: m.hostName)
+              (builtins.filter (badMachine) cfg.buildMachines)));
+        }
+      ];
+
 
     systemd.packages = [ nix ];