- Use FetchContent_Populate + add_subdirectory with EXCLUDE_FROM_ALL
  instead of FetchContent_MakeAvailable to prevent cJSON's install()
  commands from running during 'cmake --install'
- Change cJSON linkage from PUBLIC to PRIVATE since it's statically
  linked into the shared library
- Set ENABLE_TARGET_EXPORT OFF and ENABLE_CJSON_UNINSTALL OFF to further
  prevent cJSON from adding install targets
- Set ENABLE_CUSTOM_COMPILER_FLAGS OFF to prevent cJSON's strict compiler
  flags from conflicting with our project's flags
- Remove non-existent ENABLE_CJSON_INSTALL option

This fixes the release job failure where cJSON was trying to install
to /usr/local which requires elevated permissions on GitHub Actions runners.