cmake installs nlohmann_json instead of using the external package

[yurivict]

Describe the bug
CMakeLists.txt installs nlohmann-json here

Virtualy all systems have nlohmann-json packages, so you should use it like this:

find_package(nlohmann_json REQUIRED)

Installing it would cause conflicts for other packages. For example, some packages need nlohmann_json to build and they would need to remove arbor just to build themelves.

[thorstenhater]

Hi @yurivict,

thanks, this is fixed in #2481