Try specifying the nixpkgs we want like this, instead.

.circleci/config.yml

@@ -18,6 +18,10 @@ jobs:
   test:
     docker:
       - image: "nixos/nix:2.2.1"
+
+    environment:
+      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/3c83ad6ac13b67101cc3e2e07781963a010c1624.tar.gz"
+
     steps:
       - run:
           name: "Install Git"

shell.nix

-let
-  pinnedPkgs = import (builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/3c83ad6ac13b67101cc3e2e07781963a010c1624.tar.gz";
-    sha256 = "0cdq342wrkvkyccygpp1gvwp7hhqg68hljjwld4vjixm901ayy14";
-  }) {};
-in
-{ ... }:
-pinnedPkgs.mkShell {
+{ pkgs ? import <nixpkgs> { } }:
+pkgs.mkShell {
   buildInputs = [
-    pinnedPkgs.stack
+    pkgs.stack
   ];
 }