Merge pull request #5277 from NixOS/regnat/ci-better-cache-usage

flake: Use the real nixUnstable from nixpkgs

flake.nix

@@ -259,11 +259,11 @@
       # 'nix.perl-bindings' packages.
       overlay = final: prev: {
 
-        # An older version of Nix to test against when using the daemon.
-        # Currently using `nixUnstable` as the stable one doesn't respect
-        # `NIX_DAEMON_SOCKET_PATH` which is needed for the tests.
         nixStable = prev.nix;
 
+        # Forward from the previous stage as we don’t want it to pick the lowdown override
+        nixUnstable = prev.nixUnstable;
+
         nix = with final; with commonDeps pkgs; stdenv.mkDerivation {
           name = "nix-${version}";
           inherit version;