nixos: add preferLocalBuild=true; on derivations for config files

nixos/modules/security/ca.nix

@@ -14,6 +14,7 @@ let
     { files =
         cfg.certificateFiles ++
         [ (builtins.toFile "extra.crt" (concatStringsSep "\n" cfg.certificates)) ];
+      preferLocalBuild = true;
      }
     ''
       cat $files > $out

nixos/modules/security/sudo.nix

@@ -215,7 +215,10 @@ in
     environment.etc = singleton
       { source =
           pkgs.runCommand "sudoers"
-          { src = pkgs.writeText "sudoers-in" cfg.configFile; }
+          {
+            src = pkgs.writeText "sudoers-in" cfg.configFile;
+            preferLocalBuild = true;
+          }
           # Make sure that the sudoers file is syntactically valid.
           # (currently disabled - NIXOS-66)
           "${pkgs.buildPackages.sudo}/sbin/visudo -f $src -c && cp $src $out";