missed re-adding one line of this comment

.circleci/config.yml

@@ -204,6 +204,7 @@ jobs:
             # Pre-build this because doing so is somewhat memory intensive and
             # we want to turn off concurrency for this part.  We want to be
             # able to leave concurrency on for the rest of the build, though,
+            # where it doesn't cause problems and speeds things up.
             nix-build --cores 1 --max-jobs 1 \
               --arg callPackage '(import <nixpkgs> { }).callPackage' \
               ./python-challenge-bypass-ristretto.nix