Rename swift-latest symlink as swift-latest.xctoolchain
Rename swift-latest symlink as swift-latest.xctoolchain

Toolchain installers distributed on https://swift.org/download creates a
symbolic link of the toolchain as `swift-latest.xctoolchain`, but postinstall
script in this repo creates as `swift-latest`.
This patch syncs the difference of symlink name.

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