maybe like this, then

.circleci/config.yml

@@ -147,11 +147,10 @@ jobs:
 
     steps:
       - run:
-          environment:
-            NIX_PATH: "nixpkgs=https://github.com/nixos/nixpkgs/archive/7e9b0dff974c89e070da1ad85713ff3c20b0ca97.tar.gz"
           name: "Set up Cachix"
           command: |
-            nix-env -iA nixpkgs.cachix
+            nix-channel --add https://nixos.org/channels/nixos-21.05 2105-small
+            nix-env -f '<2105-small>' -i cachix
             cachix use "${CACHIX_NAME}"
 
       - "checkout"