nix-daemon: check for nixbld group before activation

Fixes #9

modules/services/nix-daemon.nix

@@ -51,5 +51,13 @@ in
         // { TMPDIR = "${cfg.tempDir}"; };
     };
 
+    system.activationScripts.nix-daemon.text = mkIf cfg.enable ''
+      buildUser=$(dscl . -read /Groups/nixbld 2>&1 | awk '/^GroupMembership: / {print $2}')
+      if [ -z $buildUser ]; then
+          echo "Using the nix-daemon requires build users, aborting activation" >&2
+          exit 2
+      fi
+    '';
+
   };
 }

modules/system/activation-scripts.nix

@@ -59,6 +59,7 @@ in
       # Prevent the current configuration from being garbage-collected.
       ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
 
+      ${cfg.activationScripts.nix-daemon.text}
       ${cfg.activationScripts.nix.text}
 
       ${cfg.activationScripts.etc.text}