Update create-gce.sh script. Set default option for GCE images to disable host key replacement by service.

(cherry picked from commit 748d96ffa3c51c3127bcdf23a88d54afad6406e9)

nixos/modules/virtualisation/google-compute-image.nix

@@ -57,6 +57,12 @@ in
   # Always include cryptsetup so that NixOps can use it.
   environment.systemPackages = [ pkgs.cryptsetup ];
 
+  # Make sure GCE image does not replace host key that NixOps sets
+  environment.etc."default/instance_configs.cfg".text = lib.mkDefault ''
+    [InstanceSetup]
+    set_host_keys = false
+  '';
+
   # Rely on GCP's firewall instead
   networking.firewall.enable = mkDefault false;