nix: fix generation of nix.conf for nix >= 2.20

(cherry picked from commit 880d9bc211)

modules/misc/nix.nix

@@ -52,13 +52,17 @@ let
       checkPhase =
         if pkgs.stdenv.hostPlatform != pkgs.stdenv.buildPlatform then ''
           echo "Ignoring validation for cross-compilation"
-        '' else ''
+        '' else
+          let
+            showCommand =
+              if isNixAtLeast "2.20pre" then "config show" else "show-config";
+          in ''
             echo "Validating generated nix.conf"
             ln -s $out ./nix.conf
             set -e
             set +o pipefail
             NIX_CONF_DIR=$PWD \
-            ${cfg.package}/bin/nix show-config ${
+              ${cfg.package}/bin/nix ${showCommand} ${
                 optionalString (isNixAtLeast "2.3pre")
                 "--no-net --option experimental-features nix-command"
               } \