nixos/tests: don't include switch-to-configuration in DUT by default

nixos/lib/testing/nixos-test-base.nix

@@ -3,7 +3,7 @@
 # even in `inheritParentConfig = false` specialisations.
 { lib, ... }:
 let
-  inherit (lib) mkForce;
+  inherit (lib) mkDefault mkForce;
 in
 {
   imports = [
@@ -22,6 +22,11 @@ in
         label = mkForce "test";
       };
     }
-
+    ({ config, ... }: {
+      # Don't pull in switch-to-configuration by default, except when specialisations are involved.
+      # This is mostly a Hydra optimization, so we don't rebuild all the tests every time switch-to-configuration-ng changes.
+      key = "no-switch-to-configuration";
+      system.switch.enable = mkDefault (config.isSpecialisation || config.specialisation != {});
+    })
   ];
 }