firefox sync-server service: make path to paster executable absolute

The systemd service was ignoring ExecStart because the path to the
paster executable was not absolute. Because ExecStart was ignored, the
service would not start.

nixos/modules/services/networking/firefox/sync-server.nix

@@ -135,7 +135,7 @@ in
           echo >> ${cfg.privateConfig} "secret = $(head -c 20 /dev/urandom | sha1sum | tr -d ' -')"
         fi
       '';
-      serviceConfig.ExecStart = "paster serve ${syncServerIni}";
+      serviceConfig.ExecStart = "${pkgs.pythonPackages.pasteScript}/bin/paster serve ${syncServerIni}";
     };
 
   };