Without `-h` option, `ln` command makes a symbolic link under the existing
link destination like `Toolchains/swift-DEVELOPMENT-SNAPSHOT-XX.xctoolchain/`.
```
$ ln -fs swift-DEVELOPMENT-SNAPSHOT-XX.xctoolchain swift-latest.xctoolchain
$ ln -fs swift-DEVELOPMENT-SNAPSHOT-YY.xctoolchain swift-latest.xctoolchain
$ tree
|-- swift-DEVELOPMENT-SNAPSHOT-XX.xctoolchain
| |-- swift-DEVELOPMENT-SNAPSHOT-YY.xctoolchain -> swift-DEVELOPMENT-SNAPSHOT-YY.xctoolchain
|-- swift-DEVELOPMENT-SNAPSHOT-YY.xctoolchain
|-- swift-latest.xctoolchain -> swift-DEVELOPMENT-SNAPSHOT-XX.xctoolchain
```
So we should not follow the existing symbolic link even if it already
exists.
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.