[TIMOB-23811] iOS: Module build should use env variable to specify path to Titanium SDK

iOS modules have a titanium.xcconfig which has a TITANIUM_SDK variable with a baked in absolute path. The problem is if the SDK moves or is updated, then this path is not updated. A better solution is to default TITANIUM_SDK to the absolute path, but then overwrite it with an environment variable set by the iOS module build with the absolute path to the current SDK used to build the module. I'm not 100% sure how to structure the xcconfig file so that the default absolute path can be overwritten by the env var, but it may be as easy as setting the TITANIUM_SDK = $(TI_SDK_PATH) in the titanium.xcconfig, then set the create a new defaults.xcconfig with a TITANIUM_SDK = /path/to/sdk and make sure the default.xcconfig is loaded before the titanium.xcconfig. If this is not possible, then we can just resolve this ticket as invalid.