[BUILD] Build break with external CMake nlohman_json package

[marcalff]

Install a nlohman_json package, for example in /opt/nlohmann_json-3.10.5

Build using:

ccmake \
-DCMAKE_PREFIX_PATH="/opt/protobuf-3.19.4;/opt/googletest-1.12.1;/opt/nlohmann_json-3.10.5;/opt/benchmark-1.8.2" \
-DBUILD_SHARED_LIBS=ON \
-DOTELCPP_PROTO_PATH=/home/malff/CODE/MY_GITHUB/opentelemetry-proto \
-DWITH_OTLP_HTTP=ON \
-DWITH_OTLP_GRPC=OFF \
-DWITH_BENCHMARK=OFF \
 ../opentelemetry-cpp

Build ... it fails:

[ 85%] Building CXX object exporters/otlp/CMakeFiles/otlp_http_log_record_exporter_test.dir/test/otlp_http_log_record_exporter_test.cc.o
/home/malff/CODE/MY_GITHUB/opentelemetry-cpp/exporters/otlp/test/otlp_http_log_record_exporter_test.cc:31:12: fatal error: nlohmann/json.hpp: No such file or directory
   31 | #  include "nlohmann/json.hpp"
      |            ^~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [exporters/otlp/CMakeFiles/otlp_http_log_record_exporter_test.dir/build.make:76: exporters/otlp/CMakeFiles/otlp_http_log_record_exporter_test.dir/test/otlp_http_log_record_exporter_test.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:5489: exporters/otlp/CMakeFiles/otlp_http_log_record_exporter_test.dir/all] Error 2
make: *** [Makefile:146: all] Error 2