Each blocks after slot with let directive captures wrong ctx

[RedHatter]

Describe the bug
When an each block follows a slot that uses a let directive that hides a variable of the same name in the parent then the each block incorrectly references the value from inside the slot.

To Reproduce
https://svelte.dev/repl/537557d31921471e97632c6780c0ea39?version=3.15.0

The first line a a shows that the initial value of the array is ['a'].
The second line b shows that the value of the array inside the slot is ['b'].
The last line a b is unexpected. It shows that the value of the array is still ['a'] yet the each block is referencing the array from the slot, ['b'].

Expected behavior
I would expect either the each block to use the parents value or for the svelte compiler to warn about let directive variable already being defined.

[Conduitry]

It looks like this was fixed on master, presumably by #3945 (the third line is now 'a a'), but it would still be good to have a test for this.