Attempt to further reduce resource usage by further reducing concurrency

.circleci/config.yml

@@ -61,7 +61,7 @@ jobs:
           name: "Build System Test Driver"
           command: |
             export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
-            nix-build --max-jobs 1 nixos/system-tests.nix -A driver
+            nix-build --max-jobs 1 --cores 1 nixos/system-tests.nix -A driver
 
           # Give it a good long while.  PaymentServer and its dependencies, in
           # particular, can take a while to build.