Fix GDScript cache assigning UID as scene path

[KoBeWi]

Fixes #82701

[reduz]

Not my area so I am not sure if this is correct, but it if the problem is that the cache needs an actual path and its getting an UID path, then this looks good.

[adamscott]

Don't be fooled by the number of lines it changes, it only adds 3 lines (the uid check). The rest is the same logic, with a different variable name.

Works well for me.

[akien-mga]

Thanks!