try to use a single nixpkgs in each job

.circleci/config.yml

@@ -456,9 +456,11 @@ jobs:
       - "checkout"
       - "run":
           name: "Generation version"
-          command: |
+          command: >-
             # The Nix package doesn't know how to do this part, unfortunately.
-            nix-shell .circleci/python.nix --run 'python setup.py update_version'
+            nix-shell .circleci/python.nix
+              --argstr pkgsVersion "$NIXPKGS"
+              --run 'python setup.py update_version'
 
       - "run":
           name: "Build and Test"

.circleci/python.nix

@@ -2,7 +2,8 @@
 let
   sources = import ../nix/sources.nix;
 in
-{ pkgs ? import sources."nixpkgs-21.11" { }
+{ pkgsVersion
+, pkgs ? import sources.${pkgsVersion} { }
 }:
 pkgs.mkShell {
   buildInputs = [