nixos/mysql: fix evaluation of percona test

Commit e14483d6a6 fixed a bug in the `ini`
type with `listsAsDuplicatedKeys = true;`: multiple list declarations
weren't merged, but latter declarations shadowed the former without any
error.

The fix brought another issue to surface however: before, the
`plugin-load-add` declaration in the MySQL test shadowed the
`auth_socket.so` setting in the module. But now the attempt
to merge a list and a single declaration breaks because of
`types.either` seeing a mix of declarations from the left
AND right type.

Turning the `plugin-load-add` in the module into a list triggers the
correct merging behavior and thus fixes the evaluation error (and
merging behavior of `plugin-load-add`)!

This wasn't an issue for mysql itself (empty `plugin-load-add` in the
test) and neither for mariadb (the `auth_socket.so` isn't added for
this).

nixos/modules/services/databases/mysql.nix

@@ -314,7 +314,7 @@ in
         binlog-ignore-db = [ "information_schema" "performance_schema" "mysql" ];
       })
       (lib.mkIf (!isMariaDB) {
-        plugin-load-add = "auth_socket.so";
+        plugin-load-add = [ "auth_socket.so" ];
       })
     ];