The previous attempt at a fix missed one important case: the types
of the values involved in the `{ Fn::Join }` expression didn't actually
match up. They all needed to be strings, but the previous implentation
just dropped list-typed values in there.

Unfortunately, there is no way to do it correctly with just string
manipulation in CloudFormation (`{ Fn::Join }` etc), so we'll have
to resort to using a Custom Resource if we encounter list values.

Actually fixes #13465.