Continue to pin nixpkgs on CI

.circleci/config.yml

@@ -30,6 +30,7 @@ jobs:
       - run:
           name: "Run Tests"
           command: |
+            export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
             nix-build nixos/unit-tests.nix && cat result
 
   build:
@@ -57,6 +58,7 @@ jobs:
       - run:
           name: "Nix Build"
           command: |
+            export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
             nix-build docs.nix
 
       - store_artifacts: