DownstreamPlaceholder::unknownCaOutput
This is a part of CA derivations that we forgot to put behind the experimental feature. This was caught by @fricklerhandwerk in https://github.com/NixOS/nix/pull/8369#discussion_r1258133719
EvalState::coerceToDerivedPath
This gives us some round trips to test. `EvalState::coerceToDerivedPathUnchecked` is a factored out helper just for unit testing.