fix(qemu-guest): let the NixOS test framework override serial-getty@ttyS0 (#362)

test-instrumentation.nix sets serial-getty@ttyS0.enable = false at normal priority to keep ttyS0 free for the test backdoor console, while qemu-guest.nix set it to true at normal priority. Two conflicting normal-priority definitions made `nix flake check` fail while evaluating packages.x86_64-linux.ruby-nixos-tests.

Use lib.mkDefault so the test framework wins in tests while ordinary QEMU guests still enable the serial getty.

modules/nixos/server/qemu-guest.nix

@@ -33,6 +33,8 @@
     services.openssh.enable = true;
     # we configure the host via nixos itself, so we don't need the cloud-init
     services.cloud-init.enable = lib.mkForce false;
-    systemd.services."serial-getty@ttyS0".enable = true;
+    # Default so the NixOS test framework can override it: test-instrumentation
+    # disables this getty to keep ttyS0 free for its backdoor console.
+    systemd.services."serial-getty@ttyS0".enable = lib.mkDefault true;
   };
 }