[5.8] Fix Eloquent UPDATE queries with alias

[staudenmeir]

#26031 qualified UPDATED_AT timestamps to fix ambiguity issues.

This doesn't work when the query uses an alias:

User::from('users as alias')->update(['foo' => 'bar']);

update `users` as `alias` set `foo` = bar, `users`.`updated_at` = 2019-05-24 20:12:50
                                            ^^^^^

Fixes #28569.