Only set GTest_SOURCE to BUNDLED on MSVC if not passed explicitly

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -63,7 +63,7 @@ set(ARROW_THIRDPARTY_DEPENDENCIES
 # supported when building with MSVC because of the way that
 # conda-forge packages have 4 variants of the libraries packaged
 # together
-if(MSVC)
+if(MSVC AND "${GTest_SOURCE}" STREQUAL "")
   set(GTest_SOURCE "BUNDLED")
 endif()