Most platforms allow override it to true anyway, but some (Linux) don't, allowing targets to set it up in their SetGlobalEnvironment(). Since you cannot do that in game/editor targets though (see UEBuildTarget.SetupGlobalEnvironment()), default value should be true to get the editor compiled with ICU.
#codereview Robert.Manuszewski
[CL 2330785 by Dmitry Rekman in Main branch]