build: adds missing config setting for vit-ss (#191)

The vit servicing station server exits silently if this configuration
parameter is missing. We set it to an empty string here because it's
passed as a flag during runtime.

Author: minikin

nix/vit-servicing-station/operables.nix

@@ -33,6 +33,7 @@
       };
       address = "0.0.0.0:8080";
       service_version = "";
+      db_url = "";
     };
   in
     std.lib.ops.mkOperable {