[10.x] Fix cursor paginate with union and column alias

[thijsvdanker]

When using cursorPaginate on a query with a union the where clause on the union uses the wrong column name.

Table Post:

id
start_time

Table News:

id
created_at

Post::toBase()->select('id', 'start_time as created_at')->union(News::toBase()->select('id', 'created_at'))->orderBy('created_at')->cursorPaginate(10, ['created_at']);

Should sort Post by the start_time column and News by thecreated_at column.

The current implementation sorts both Post and News by the start_time column.

This is similar to #50796 but applied against 10.x

[taylorotwell]

It concerns me we are just straight up changing existing tests.