build: Let CMake use FindBoost.cmake first (#909)

Increases compatibility right now. We will reconsider when we can drop CMake 3.14 whether to use `CMAKE_FIND_PACKAGE_PREFER_CONFIG1`, as discussed in #886.

CMakeLists.txt

@@ -140,7 +140,8 @@ set(_acts_tbb_version 2020.1)
 if (ACTS_USE_SYSTEM_BOOST)
   # NOTE we use boost::filesystem instead of std::filesystem since the later
   # is not uniformly supported even on compilers that nominally support C++17
-  find_package(Boost ${_acts_boost_version} REQUIRED CONFIG COMPONENTS filesystem program_options unit_test_framework)
+  # NOTE FindBoost.cmake looks for BoostConfig.cmake first, before running it's own logic.
+  find_package(Boost ${_acts_boost_version} REQUIRED COMPONENTS filesystem program_options unit_test_framework)
 else()
   add_subdirectory(thirdparty/boost)
 endif()