livebook: Set KillMode=mixed

This will gracefully shut down the service instead of resulting in errors like
this:

```
Jan 24 10:11:11 foo livebook[981676]: 10:11:11.922 [error] GenServer :disksup terminating
Jan 24 10:11:11 foo livebook[981676]: ** (stop) {:port_died, :normal}
Jan 24 10:11:11 foo livebook[981676]: Last message: {:EXIT, #Port<0.8>, :normal}
Jan 24 10:11:11 foo livebook[981676]: 10:11:11.922 [error] GenServer :memsup terminating
Jan 24 10:11:11 foo livebook[981676]: ** (stop) {:port_died, :normal}
```

nixos/modules/services/development/livebook.nix

@@ -88,6 +88,7 @@ in
         Restart = "always";
         EnvironmentFile = cfg.environmentFile;
         ExecStart = "${cfg.package}/bin/livebook start";
+        KillMode = "mixed";
       };
       environment = mapAttrs (name: value:
         if isBool value then boolToString value else toString value)