`uncons` and `concurrently` are not friends

[armanbilge]

... but maybe they can be :) see context in https://github.com/typelevel/fs2/pull/3112#pullrequestreview-1263803863.

One solution I experimented with in https://github.com/typelevel/fs2/pull/3112#issuecomment-1399139450 was when compiling the background stream in concurrently, instead of giving it a new root scope it should derive its scope from the foreground stream.