Fsharp non-deterministically fails to download vswhere

I saw this failure couple times in the past. The recent VMR build failed because of it.

```
Downloading vswhere
Stream was not readable.
System.ArgumentException: Stream was not readable.
at System.IO.StreamReader..ctor(Stream stream, Encoding encoding, Boolean detectEncodingFromByteOrderMarks, Int32 bufferSize, Boolean leaveOpen)
at System.IO.StreamReader..ctor(Stream stream, Encoding encoding)
at Microsoft.PowerShell.Commands.FileSystemContentReaderWriter.CreateStreams(String filePath, String streamName, FileMode fileMode, FileAccess fileAccess, FileShare fileShare, Encoding fileEncoding)
at Microsoft.PowerShell.Commands.FileSystemProvider.GetContentWriter(String path)
at EnablePreviewSdks, D:\a\_work\1\s\src\fsharp\eng\build.ps1: line 517
at , D:\a\_work\1\s\src\fsharp\eng\build.ps1: line 544
at , : line 1
at , : line 1
##[error](NETCORE_ENGINEERING_TELEMETRY=Build) Failure preparing build
```

https://dev.azure.com/dnceng/internal/_build/results?buildId=2676220&view=logs&j=89eea50f-59b5-5921-c3ce-ce40b36fa819&t=483e1309-93e4-5a7a-74de-93b8d658274f