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

(cherry picked from commit 880d9bc211)

modules/misc/nix.nix

@@ -52,22 +52,26 @@ let
       checkPhase =
         if pkgs.stdenv.hostPlatform != pkgs.stdenv.buildPlatform then ''
           echo "Ignoring validation for cross-compilation"
-        '' else ''
-          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 ${
-              optionalString (isNixAtLeast "2.3pre")
-              "--no-net --option experimental-features nix-command"
-            } \
-            |& sed -e 's/^warning:/error:/' \
-            | (! grep '${
-              if cfg.checkConfig then "^error:" else "^error: unknown setting"
-            }')
-          set -o pipefail
-        '';
+        '' 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 ${showCommand} ${
+                optionalString (isNixAtLeast "2.3pre")
+                "--no-net --option experimental-features nix-command"
+              } \
+              |& sed -e 's/^warning:/error:/' \
+              | (! grep '${
+                if cfg.checkConfig then "^error:" else "^error: unknown setting"
+              }')
+            set -o pipefail
+          '';
     };
 
   semanticConfType = with types;