circleci: Bump down thread count

It appears that our jobs generally run on VMs with 2 vCPUs. Consequently running
with 8 jobs will completely oversubscribe the machine. I suspect this is the
cause of #14453. Let's bump this down to 3 for now. Ideally we would determine
this from the environment.

.circleci/config.yml

@@ -22,7 +22,7 @@ aliases:
       name: submodules
       command: .circleci/fetch-submodules.sh
   - &buildenv
-    THREADS: 8
+    THREADS: 3
     SKIP_PERF_TESTS: YES
     VERBOSE: 2
   - &boot