[5.8] Fix fromSub() and joinSub() with table prefix

[staudenmeir]

The subquery alias in fromSub() and joinSub() has to include the table prefix:

'sqlite' => [
    'driver' => 'sqlite',
    'database' => ':memory:',
    'prefix' => 'prefix_',
],

DB::table('users')->joinSub(DB::table('posts'), 'alias', 'alias.user_id', 'users.id')->get();

// expected
select * from "prefix_users"
inner join (select * from "prefix_posts") as "prefix_alias"
on "prefix_alias"."user_id" = "prefix_users"."id"

// actual
select * from "prefix_users"
inner join (select * from "prefix_posts") as "alias"
on "prefix_alias"."user_id" = "prefix_users"."id"

Fixes #28391.