Diagnostic shows wrong type in unpack assignment

The diagnostic message in the following example should say *"Object of type **`int`** is not assignable to `str`"*:
```py
x: str = ""

def returns_tuple() -> tuple[int, int]:
    return (0, 0)

# Object of type `tuple[int, int]` is not assignable to `str`
x, y = returns_tuple()
```
https://play.ty.dev/29ce53bf-0300-4f30-b00d-c30480f7f497