Use `networking.fqdn` to define `targetHost`, rather than constructing it ourselves.

While working on #84, I discovered that we could use networking.fqdn.

morph/lib/base.nix

@@ -31,7 +31,7 @@
     # being configured and using variable names complicates a lot of things).
     # Instead, just tell morph how to reach the node here - by using its fully
     # qualified domain name.
-    deployment.targetHost = "${config.networking.hostName}.${config.networking.domain}";
+    deployment.targetHost = config.networking.fqdn;
 
     assertions = [
       # This is a check to save somebody in the future trying to debug why