feat(midnight): limit concurrent builds

We've overloading midnight with our CI - let's stop ourselves from
trying to do more things than midnight supports at once...

packetmix/systems/midnight/lix.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  nix.settings = {
+    max-jobs = 8;
+    cores = 1;
+  };
+}