Current AppVeyor image includes a very old vcpkg snapshot. Because of
that, this doesn't include https://github.com/Microsoft/vcpkg/pull/5258
specifically
https://github.com/Microsoft/vcpkg/pull/5258/commits/c895bb658d52bee471169460c711fdd90e3d5b7b

Because of that, find_package only works for find_package(nlohmann-json)
but not find_package(nlohmann_json). Updating the ports cache should fix
that.