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

nixos/modules/services/databases/mysql.nix

@@ -334,6 +334,12 @@ in
 
     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";