nixos/nix-daemon: don't refer to deprecated option aliases

The `gc-` prefixed versions of these options have been
deprecated since Nix 2.0.

nixos/modules/services/continuous-integration/hydra/default.nix

@@ -242,8 +242,8 @@ in
     environment.variables = hydraEnv;
 
     nix.extraOptions = ''
-      gc-keep-outputs = true
-      gc-keep-derivations = true
+      keep-outputs = true
+      keep-derivations = true
 
       # The default (`true') slows Nix down a lot since the build farm
       # has so many GC roots.

nixos/modules/services/misc/nix-daemon.nix

@@ -152,8 +152,8 @@ in
         type = types.lines;
         default = "";
         example = ''
-          gc-keep-outputs = true
-          gc-keep-derivations = true
+          keep-outputs = true
+          keep-derivations = true
         '';
         description = "Additional text appended to <filename>nix.conf</filename>.";
       };