FindBoost: Fix release name candidate list construction
Changes in !639 (merged) accidentally left a
set() instead of a
list(APPEND) while constructing
_boost_RELEASE_NAMES. Fix the logic to match what was done for
_boost_DEBUG_NAMES. Otherwise we drop some of the candidate names.
Boost_USE_DEBUG_RUNTIME is OFF,
_boost_RELEASE_NAMES will be cleared at the line so that fail to find library when configure for Visual Studio.