[build-windows] Don't download ICU
[build-windows] Don't download ICU

...until we want to build Foundation.

apple/swift,glessard/swift,benlangmuir/swift,apple/swift,apple/swift,roambotics/swift,glessard/swift,glessard/swift,roambotics/swift,apple/swift,roambotics/swift,glessard/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,benlangmuir/swift,apple/swift,roambotics/swift,apple/swift,roambotics/swift