nix: core: set usable Nix version

`latest` makes "devenv" hang forever for some reason, see
cachix/devenv#1364 for details,
while more dated versions either lack essential features,
including experimental ones, or works overall strangely.

modules/nix/core/default.nix

@@ -74,7 +74,7 @@ in
           trusted-public-keys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];
           trusted-users = [ "root" user ];
         };
-        package = pkgs.nixVersions.latest;
+        package = pkgs.nixVersions.nix_2_22;
         extraOptions = ''
           auto-optimise-store = true
           keep-outputs = true