FindBoost: Avoid leaking internal Boost_ROOT setting

Since commit 4b2a61946f (FindBoost: Honor BOOST_ROOT when finding
upstream BoostConfig, 2020-10-22, v3.19.0-rc2~14^2) we accidentally leak
our internal setting of `Boost_ROOT`.  Revise the logic to unset our
temporary value.

Fixes: #21379

Modules/FindBoost.cmake

@@ -450,13 +450,19 @@ if (NOT Boost_NO_BOOST_CMAKE)
   # Do the same find_package call but look specifically for the CMake version.
   # Note that args are passed in the Boost_FIND_xxxxx variables, so there is no
   # need to delegate them to this find_package call.
-  cmake_policy(PUSH)
   if(BOOST_ROOT AND NOT Boost_ROOT)
+    # Honor BOOST_ROOT by setting Boost_ROOT with CMP0074 NEW behavior.
+    cmake_policy(PUSH)
     cmake_policy(SET CMP0074 NEW)
     set(Boost_ROOT "${BOOST_ROOT}")
+    set(_Boost_ROOT_FOR_CONFIG 1)
   endif()
   find_package(Boost QUIET NO_MODULE ${_boost_FIND_PACKAGE_ARGS})
-  cmake_policy(POP)
+  if(_Boost_ROOT_FOR_CONFIG)
+    unset(_Boost_ROOT_FOR_CONFIG)
+    unset(Boost_ROOT)
+    cmake_policy(POP)
+  endif()
   if (DEFINED Boost_DIR)
     mark_as_advanced(Boost_DIR)
   endif ()