nixos/nebula: Always restart

Without this, if the network goes down for a while, systemd will give up after 5 restarts:

    Scheduled restart job, restart counter is at 5.
    Stopped Nebula VPN service for myvpn.
    nebula@myvpn.service: Start request repeated too quickly.
    Failed with result 'exit-code'.
    Failed to start Nebula VPN service for myvpn.

Most network services need this, but for VPNs it's extra important.

nixos/modules/services/networking/nebula.nix

@@ -192,6 +192,7 @@ in
                 Group = networkId;
               })
             ];
+            unitConfig.StartLimitIntervalSec = 0; # ensure Restart=always is always honoured (networks can go down for arbitrarily long)
           };
         }) enabledNetworks);