Fix tar on Windows

[GerardSmit]

When the SDK doesn't exists in $(WasiSdkRoot) it's getting downloaded and extracted by tar.

However, I'm getting the following error when I run dotnet build

error MSB3073: The command "tar -xf "C:\Users\X\AppData\Local\Temp\51xb0ufs.jsz\wasi-sdk-24.0-x86_64-windows.tar.gz" -C . --strip-components=1" exited with code 128

This is because tar is overridden by Git on Windows on my machine:

$ where tar
C:\Program Files\Git\usr\bin\tar.exe
C:\Windows\System32\tar.exe

So, Git on Windows tar is being used, and for some reason, it doesn't work.

In this PR I've changed that the built-in Windows tar.exe is always used, which does work.

Not sure if this fixes the following issue: #87