tests.nixos-functions: don’t omit derivations on non‐Linux

Hydra can handle packages that `throw` but not attributes that
disappear depending on platform, and it seems okay for evaluation to
fail here for non‐Linux platforms.

pkgs/test/nixos-functions/default.nix

···
     versionSuffix = "test";
     label = "test";
   };
-in lib.optionalAttrs stdenv.hostPlatform.isLinux (
-  pkgs.recurseIntoAttrs {
+in pkgs.recurseIntoAttrs {
 
-    nixos-test = (pkgs.nixos {
-      system.nixos = dummyVersioning;
-      boot.loader.grub.enable = false;
-      fileSystems."/".device = "/dev/null";
-      system.stateVersion = lib.trivial.release;
-    }).toplevel;
+  nixos-test = (pkgs.nixos {
+    system.nixos = dummyVersioning;
+    boot.loader.grub.enable = false;
+    fileSystems."/".device = "/dev/null";
+    system.stateVersion = lib.trivial.release;
+  }).toplevel;
 
-  }
-)
+}