Adjust core count - job count is in gitlab-runner.toml though.

darwin/ghc-gitlab-ci.nix

@@ -32,6 +32,6 @@ stdenv.mkDerivation {
     export HOME="/Users/builder"
     export PATH="${pkgs.clang_10}/bin:$PATH"
     export FONTCONFIG_FILE=${fonts}
-    export CORES=12
+    export CORES=4
     '';
 }