nixos/mysql: run as mysql user and group

As we don't need to setup data directories from ExecStartPre= scripts
anymore, which required root, but use systemd.tmpfiles.rules instead,
everything can be run as just the mysql user.

nixos/modules/services/databases/mysql.nix

@@ -326,6 +326,8 @@ in
         '';
 
         serviceConfig = {
+          User = cfg.user;
+          Group = "mysql";
           Type = if hasNotify then "notify" else "simple";
           # /run/mysqld needs to be created in addition to pidDir, as they could point to different locations
           RuntimeDirectory = "mysqld";