Tweak the CircleCI build configuration for better performance

.circleci/config.yml

@@ -337,7 +337,17 @@ jobs:
       - "run":
           name: "Build and Test"
           command: |
-            nix-build nix/
+            # CircleCI build environment looks like it has a zillion and a
+            # half cores.  Don't let Nix autodetect this high core count
+            # because it blows up memory usage and fails the test run.  Pick a
+            # number of cores that suites the build environment we're paying
+            # for (the free one!).
+            #
+            # Also, let it run more than one job at a time because we have to
+            # build a couple simple little dependencies that don't take
+            # advantage of multiple cores and we get a little speedup by doing
+            # them in parallel.
+            nix-build --cores 3 --max-jobs 2 nix/
 
   # Generate up-to-date data for the dependency graph visualizer.
   build-porting-depgraph: