Specify a nixpkgs to use

.circleci/config.yml

@@ -54,8 +54,25 @@ jobs:
 
     environment:
       CODECOV_TOKEN: "cc6e4697-4337-4506-88af-92b8f8ca6b22"
+      # Specify a revision of NixOS/nixpkgs to run against.  This essentially
+      # pins the majority of the software involved in the build.  This
+      # revision is selected arbitrarily.  It's somewhat current as of the
+      # time of this comment.  We can bump it to a newer version when that
+      # makes sense.  Meanwhile, the platform won't shift around beneath us
+      # unexpectedly.
+      NIXPKGS_REV: "5d5cd70516001e79516d2ade8bcf31df208a4ef3"
 
     steps:
+      - run:
+          # Get NIX_PATH set for the rest of the job so that the revision of
+          # nixpkgs we selected will be used everywhere Nix pulls in software.
+          # There is no way to set an environment variable containing the
+          # value of another environment variable on CircleCI except to use
+          # the `BASE_ENV` feature as we do here.
+          name: "Setup NIX_PATH Environment Variable"
+          command: |
+            echo "export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$NIXPKGS_REV.tar.gz" >> $BASH_ENV
+
       - "checkout"
 
       - run: