nixos/mysql: Support bootstrapping a Galera cluster

The default galera_new_cluster script tries to set this environment
variable using systemctl set-environment which doesn't work if the
variable is not being used in the unit file ;)

nixos/modules/services/databases/mysql.nix

@@ -274,7 +274,8 @@ in
         serviceConfig = {
           Type = if hasNotify then "notify" else "simple";
           RuntimeDirectory = "mysqld";
-          ExecStart = "${mysql}/bin/mysqld --defaults-file=/etc/my.cnf ${mysqldOptions}";
+          # The last two environment variables are used for starting Galera clusters
+          ExecStart = "${mysql}/bin/mysqld --defaults-file=/etc/my.cnf ${mysqldOptions} $_WSREP_NEW_CLUSTER $_WSREP_START_POSITION";
         };
 
         postStart = ''