build-unit-test-docker: switch json to use cmake

nlohmann/json supports CMake and we use it in the bitbake
recipe, so use it here as well.  The bitbake recipe has a
'install_append' to symlink the final header directly into
'{includedir}/json.hpp' so we need to continue to do the
same here.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ibf61a7a471216c43427109907a54e81dde1ae8fc
1 file changed