Investigate korio writing file in iOS

https://kotlinlang.slack.com/archives/CJEF0LB6Y/p1633938221093600

```kotlin
val vfsFile = localVfs(getRootDir())["file.txt"]
vfsFile.writeString("Hello World!")

override fun getRootDir(): String {
val nsUserDomainMask = 1UL
val nsCachesDirectory = 13UL
val cacheDirectoryList = NSFileManager.defaultManager().URLsForDirectory(
nsCachesDirectory,
nsUserDomainMask
)

return (cacheDirectoryList.first() as? NSURL)?.path() ?: ""
}
```

Start by reproducing the sample using localVfs(getRootDir()), vfsFile.writeString("Hello World!"), and the NSFileManager-based getRootDir() shown in the issue. Trace the iOS file-writing path and document or test the expected successful write behavior once the cause is identified.

Written by the indexing model from the issue text.