nixos: Use checks instead of extraDependencies

... as appropriate.

This drops a few unnecessary store paths from the system closure.

nixos/modules/security/wrappers/default.nix

···
         '';
 
     ###### wrappers consistency checks
-    system.extraDependencies = lib.singleton (pkgs.runCommandLocal
+    system.checks = lib.singleton (pkgs.runCommandLocal
       "ensure-all-wrappers-paths-exist" { }
       ''
         # make sure we produce output

nixos/modules/services/databases/postgresql.nix

···
      "/share/postgresql"
     ];
 
-    system.extraDependencies = lib.optional (cfg.checkConfig && pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) configFileCheck;
+    system.checks = lib.optional (cfg.checkConfig && pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) configFileCheck;
 
     systemd.services.postgresql =
       { description = "PostgreSQL Server";

nixos/modules/services/web-servers/varnish/default.nix

···
     environment.systemPackages = [ cfg.package ];
 
     # check .vcl syntax at compile time (e.g. before nixops deployment)
-    system.extraDependencies = mkIf cfg.enableConfigCheck [
+    system.checks = mkIf cfg.enableConfigCheck [
       (pkgs.runCommand "check-varnish-syntax" {} ''
         ${cfg.package}/bin/varnishd -C ${commandLine} 2> $out || (cat $out; exit 1)
       '')

nixos/modules/services/x11/xserver.nix

···
         xorg.xf86inputevdev.out
       ];
 
-    system.extraDependencies = singleton (pkgs.runCommand "xkb-validated" {
+    system.checks = singleton (pkgs.runCommand "xkb-validated" {
       inherit (cfg) xkbModel layout xkbVariant xkbOptions;
       nativeBuildInputs = with pkgs.buildPackages; [ xkbvalidate ];
       preferLocalBuild = true;