nixos/mysql: fix permission error during first startup (#331296)

    environment.etc."my.cnf".source = cfg.configFile;

    # The mysql_install_db binary will try to adjust the permissions, but fail to do so with a permission

    # denied error in some circumstances. Setting the permissions manually with tmpfiles is a workaround.

    systemd.tmpfiles.rules = [

      "d ${cfg.dataDir} 0755 ${cfg.user} ${cfg.group} - -"

    ];

    systemd.services.mysql = {

      description = "MySQL Server";