nixos/mysql: Explicitly set datadir in my.cnf

While this seems silly at first (it's already given as start parameter
to mysqld), it seems like xtrabackup needs that sometimes.
Without it, a Galera cluster cannot be run using the xtrabackup
replication method.

nixos/modules/services/databases/mysql.nix

@@ -23,6 +23,7 @@ let
   ''
     [mysqld]
     port = ${toString cfg.port}
+    datadir = ${cfg.dataDir}
     ${optionalString (cfg.bind != null) "bind-address = ${cfg.bind}" }
     ${optionalString (cfg.replication.role == "master" || cfg.replication.role == "slave") "log-bin=mysql-bin"}
     ${optionalString (cfg.replication.role == "master" || cfg.replication.role == "slave") "server-id = ${toString cfg.replication.serverId}"}